I just built my 283 and used 601HO heads used on Z28..Found them on e-bay rebuilt for $150 ea...They had a lot of new valves in them. Put Z28 spring on them..intake 1.84 in and exhaust 1.50. Good flow for my little 283 and 58cc combustion chamber.
I haven't looked up the casting number so I assuming that Z28 engine that donated those heads was a 305. That valve size and combustion chamber volume were only used on the 305 that had baby sized intake ports. The ports were about the size of Power Pack heads but raised up like a Vortec head. The valves are bigger than a Power Pack head off of a 283, but the small chamber combined with flat top pistons is going to have you burning 92 octane pump gas.
My experience is with one set of aftermarket heads. The engine was a 406 with a stock bottom end. The crank was cut .010 on the rods and mains, deck was cut .020", stock rods resized on the big end and Speed Pro dished pistons installed on the rods. In stock form the engine made about 265 hp 400 ft lbs tq @2400 rpm at the flywheel.
on that short block I added AFR 195ST heads right out of the box, A Lunati cam with 110° lobe separation, .489/.504 lift, 268/276 duration. A set of 1.6 rockers were added raising the lift to .521/.538. Then the normal stuff like headers, a Performer manifold and a Quadrajet carb. On a chassis dyno reading at the rear wheels was 370 hp and 474 lb ft of torque. Adjusted to the flywheel, approximately 430 hp and 520 lb ft of torque. This was with a stock rotating assembly, heads and cam made the difference. Can't say which had the most effect but the heads did a fair share of the increases.
So, here is a 4 year update. While I bought the Motown 220 heads, I still have not installed them. I kept my engine as is. If I ever build a 383, however, I will use the Motowns, so that I can build the entire engine around the heads. I have been stockpiling parts since this thread. So far, I have Performer RPM Airgap I bought used, a set of Hooker headers, as well as I found an Edelbrock 750 1407 I had bought 20 years ago. I thought the carb was lost, but when I was cleaning my dad's house a couple years ago, I found it. So, I have a whole top of an engine ready when I do want to build a 383. But since I still have low miles on my 355, I can't justify a rebuild just yet. Maybe in a few years, or maybe if something catastrophic happens. Right now, I am happy with my 355.
Now, I am taking the advice from all of you, in that the biggest gain is seen with gearing. So,I'm swapping out my TH350 with a 700R4! That should be a nice fit with my 3.42 gears. Putting that tax refund to work, HAHA! Just waiting on my friend and transmission builder to finish a few jobs, and will be installing in 2 weeks. I can't wait!