You could replace your old 250 six with a newer Atlas six out of a Chevy light truck or a Jeep.
The DOHC fuel injected 254 cubic inch six adapts well to turbo charging. Since it is turbo charged a lot of the energy in the exhaust is transferred to the compressor so you can run straight pipes for added noise to get the sound you want. More power and noise while retaining a straight six format.
The six you have now has a 3.875" bore and a 3.53" stroke so if you had two more cylinders it would displace 328 cubes. A SBC 327 would blow it away however because the SBC breathers much better than any configuration of six cylinder head (even the Atlas which is much improved over the old 250). Power lies in the heads, always! want more power improve the heads ability to breath. This is why people who race the Chevy in line six saw two aftermarket aluminum SBC heads into pieces that are then welded back together to form a long six cylinder head with SBC ports and valves.
Cheapest solution to your problem is a SBC 350 out of a 1996-2004 Chevy pick-up converted over to a carb and a new Vortec intake manifold. You will also need a non-computer controlled HEI distributor, and a thicker (four core) radiator. Then it can be bolted in place with two new motor mounts. To complete your conversion include two glass packs for your exhaust.
Other than including nitrous there is no easy or cheap way for a Chevy in-line six to make more power. GMC offered a 502.7-cubic-inch (8.2 L); 4 9⁄16 by 5 1⁄8 inches (115.9 mm × 130.2 mm) GMC inline six. It was offered starting in 1952 and ending with the 1959 model year installed in GMC 630, 660, 720, and 750 series semi trucks. This engine was as big as and as heavy as a modern Cummins diesel engine and weighed as much so it isn't really an option. GMC offered a 478 cubic inch V6 gas engine installed in pick-up trucks, but it was also as large as a BBC which will fit in your 1976 Nova (the BBC not the GMC V6).
Problem is with the head since the rest of the engine is a 327 in a different form that is missing two cylinders. Jaguar and BMW make a straight six that will blow away a SBC in class racing (the 259 cube XK6 Jags beat the 327 Corvette like a rug in racing), Today's BMW DOHC straight six makes 280 horsepower out of 210 cubic inches, so a straight six isn't what limits your ability to make power but the head that Chevy cast to put on top of it's six that is killing your aspirations.
The straight six was designed for low speed torque with excellent fuel economy. Since GM policy says you are allowed only one part instead of two they designed the straight six using SBC parts, but topped the motor with a cast iron (Jag and BMW use an aluminum head that shaves 70 pounds of weight from the front end) head that shares ports to reduce cost (it was going into an economy car).
The intake ports, (there are three) have a huge opening that is blocked by an equally large cast iron boss that seals the head bolt from a vacuum leak or coolant leak. This boss effectively obstructs the port preventing air to flow rapidly into the engine without separating the fuel from the air flow. The valves are small in diameter and the stock cam keeps the power band low to match the air flow potential.
An Otto cycle engine makes it's power in high RPM operation (this is why there are no Diesel powered F1 cars, and why the Brayton cycle STP Special jet turbine was made illegal), because it is all about the number of power strokes per minute.
This power is the result of burning fuel that requires air to burn it. You can pour a five gallon can of gas into your six cylinder engine, but it will die a horrible death if you do so. An engine runs on gas fumes suspended in air not liquid gas. So fuel isn't the limiting factor, but getting air in is.
This is why people run a turbo charger or NOS on their engines. Because it is the lack of air that restricts the power an engine can make.
NOS breaks down into nitrogen dioxide and oxygen when heated. That extra oxygen is what gives your engine it's boost in power (provided you also add more fuel). Any blower adds more oxygen by pumping in more air than the engine can suck in through it's heads by itself. The only difference is in how you power the blower (compressor) with most being belt driven off the crank and a turbo using the wasted energy of the heat of combustion to power the compressor (no parasitic drain on engine horsepower to drive it).
You can turbocharge your six for more power, but since you can not reuse the plumbing required later on a V8 I advise against it. You can add a Paxton style scroll compressor to your six and aside from brackets it can later be reused on a V8 application. Same for a roots style blower though you would have to fabricate your own intake manifold to mount it in addition to brackets for the drive belt (I have seen two of these locally at car shows). I mention reusing these power adders on a V8 because even with a power adder you still will be hobbled by forcing air through a restrictive L6 head; and after being beaten by many similar normally aspirated V8 powered cars you will want a V8 as well.
Good news is your car will accept a SBC or a BBC or a Ls-x engine so you can drive as fast as you can afford.
Leo has a Studebaker powered by a 292 bread truck six with a welded SBC head on top to make lots of power to win consistently in his CLASS of racing. Still won't beat a BBC powered dragster in the quarter unless the digger driver is asleep at the tree.
One thing you can do with any inline six is to replace the stock muffler with a straight thru muffler (like a cherry bomb).
Sound won't be that obtrusive, but will definitely take on a much better attitude.
Find the longest one you can (probably around 3 feet, they used to make 4 footers but I've not seen one in a very long time) and you'll end up with a nice mellow rumble.
Larger diameter exhaust pipe will help with that, too.
Trade up from the stock one barrel carb to a two barrel or a 4 barrel.
Chevy used to make a 2-bbl. integral intake cylinder head for the trucks, '79-'84.
Accepted a stock 2G.
Along with the truck exhaust manifold that engine was rated by the factory @ 120 HP and 200 ft.lbs. of torque (up from 110/185 for the older manifold/carb setup).
However, if you're trying to do this on the cheap, you might want to consider a carb adapter that allows you to mount a 2-bbl carb on a 1-bbl. plenum.
Clifford used to offer those (those are the people for later inline six performance parts!)
There is also a nice little marine cam that was made for the 194-250 engines.
Not hard to live with, increases power by about 15-17 hp.
Mellings makes a good copy of that cam (Schnieder 252-56H is another good copy).
Cheapest and probably easiest would be the muffler and cam swap.
It'll sound better and pull a little harder.
Slightly shorter tires will make the engine believe its pulling shorter gears, too.
Maybe changing from "75" series to "70" series tires will give you a little extra grunt...just remember that it will throw the speedo/odo off a little, too.
Try looking at Clifford industries.They used to sell 4 barrel intakes and header exhaust kits.Just make sure your motor is healthy enough to take the extra flow.I believe you can run duals.I would use turbo mufflers.It would make a good rumble.
Or you can drive it as is with just a tranny change. I swapped out my three in the tree for a floor mounted four speed Muncie (this is the one time I would recommend the Saginaw cast iron case four speed) in my 194 powered '67 Chevy II. I raced every Friday night at Twin Cities Drag Strip so that I became so consistent that I beat all comers. With a little practice you can put a Hemi powered '70 Challenger on the trailer. Seriously I beat the owner of the race track's wife so many times with her Challenger that she forced the owner to ban me from the track.
The four speed helps performance the same way a sixteen speed Spicer tranny in a semi tractor can get as small engine to pull tremendous loads. The more gears you have the better chance you have of keeping your engine near it's power band peak for better performance. The 1966-'70 four speed wide ratio box has gear ratios of:
While your Saginaw three speed has gear ratios of:
So it is that third gear that helps keep the car accelerating instead of falling down.
Once again you will probably want to think of the future as the Great Carnac sees a V8 in your car down the road as you are already unhappy with the amount of power it makes. This is were I say buy a Muncie over the cheaper Saginaw but with modern OD trannies you may not need it in the future.
I had the same problem when I was younger and put a 427 BBC in my '67 Chevy II.
That's only one variation of the Sagniaw, Dave.
There was also a 3.5/1.89 and 3.11/1.84 gearset, as well as the standard truck gearset; 2.85/1.68.
GM took those 3-peed gearsets and developed 4-speed gearsets from them, too.
Problem with a Saginaw is its a notchy transmission.
Hard to speed shift.
The Muncie was much better at doing that.
Here's a link showing the specs on the Saginaw's, if anyone's interested...https://www.advanceadapters.com/tech-vault/4-saginaw-3--4-speeds/
You can turbocharge any engine, but is the cost worth the return (ROI in business speak). You can find a used 305 for the price of it's scrap value ($8.70 per hundred weight). This will nearly double your power level, for the price of the conversion (motor mounts and exhaust pipes and a bigger radiator).
Once you have a SBC under the hood up and running you can now begin saving for a bigger beter SBC (I'd recommend a Blue Print SBC 400). If that doesn't feed your need for speed (and you can easily exceed the performance of a BBC396 with a SBC 400 at half the costs) then you can think about turboshargers et. al.)
I think you will be very happy with just a SBC 400 while retaining your 305 air cleaner and decal.
I agree with Dave here, for the reason of how well the head will seal.
The inline sixes were only made with two rows of head bolts, yielding 4 bolts / cylinder.
With the V8's, you have 5 bolts / cylinder, due to an extra row of head bolts that run down the middle of the cylinders.
One extra bolt might not seem like much, but it is paramount for keeping the cyliinder sealed when the pressure is increased (and that's how a turbo works. It increases cylinder pressure).
Not saying you absolutely can't turbo a 250 (there's a series of youtube videos on that very subject), but if you're hot on a turbocharged engine, you're better off using one of the V8's, whether big block or small.
That being said, a mildly modified inline six can be a lot of fun to drive, especially in a light car like a Nova.