I will be the first to criticize vendors for making generic headers and then charging custom header prices, but to be fair they do make their headers to fit the greatest number of applications possible based upon certain assumptions if they are to stay in business. Where a lot of people have a problem is when they have an application were a generic header will not fit because they do not have the same installation that the factory used for that application ( and they built the generic header anticipating finding a factory install). In other words they actually need a custom header to fit a generic situation because they did something the vendor didn't anticipate; namely using SBC frame pedestal mounts or the incorrect year motor mounts (they're different thickness for the 1967-'68 year cars, than they are for 1969 and up cars), and equipping their motors with later model year power steering and A/C brackets. This is were a lot of the conflicts arise.
If you want to keep the SBC mounts more power to you it keeps the motor lower in the chassis for a better roll center, and further back for better weight transfer. Of course the HEI you want to use won't fit without taking a sledge to the fire wall and it wouldn't hurt to put a few dimples there for bolt clearance to get the bell housing on and off the car. You will also have to grind slots in either the frame rails or the cross member to get the tranny to mount were it belongs, but it can be done. Then there will be that pesky header clearance problem. You can do what hundreds of back yard mechanics have done before you; pound the tubes half closed for clearance and grind down the two inner bolt heads of the power steering gear box (manual has more room) to keep from punching a hole in the header tube. (Ignore the extra heat you're pumping into the gear box, your predecessors always have, and they never lived to tell of any problems with steering.)
Hopefully you will never have a bearing problem or need to drop the pan to inspect the oil pump because the oil pan will be resting on the front cross member. Drag cars don't need a sway bar, which is lucky, because the harmonic balancer will be right in the same location as it would be if it were run on a small block (the BBC sway bar is of a larger diameter and has a crock bent into the front of the bar to clear the balancer and belts).
Getting a fan shroud to work will require finding and modifying a SBC A/C fan shroud because the BBC engine stands shift everything to the passenger side including the opening in the fan shroud for the fan. The SBC with A/C fan shroud is the right width to clear the 23 inch wide four core radiator, but is centered. It is also going to be too long (by about 3/4 inch).
I personally have performed about forty such shade tree conversions for customers who wanted BBC power in a Camaro or more frequently the Nova as it would be more of a sleeper. It was never a problem in the good old days because you used to be able to order headers specifically for SBC motor mount BBC conversions (why Headman and Hooker stopped making this valuable product I have no idea, though I suspect the number of sales dropped below production costs). My customers were yanking heater's and swapping in four core radiators out of Impalas (they are two inches taller than the Nova/Camaro, but if you lay it on the subframe and use strapping on top to hold it in place it runs cooler). Fineness or concurs wasn't even a consideration back then, numbers matching? Matching what? Most of the cars I worked on back then were L-6 econo boxes without sway bars or A/C to get in the way (Brakes. We were more concerned about getting going as fast as we could instead of thinking about stopping (that cost at least six associates who I worked with their lives so live and learn).
Big Dave
