You need to read the footnotes towards the back of the paper catalog (I kept some mid seventies, that is to say just Hooker and Headman catalogs), as they will tell you the exclusions. On line the adds hype the big print giveth by saying: "It will fit!"
, but the small print; which is the foot notes in the back of the catalog, "taketh away" (NO it won't fit if you have any of these accessories or heads/car combinations!)
I gave up on the over the counter header companies as they have one model that fits every Chevy Car body made with-in the same series (example '67-'69 Camaro, which excludes 1970-'81, 1982-'92, 1993-'02, 2010-'15, and 2016 and up). That one statement excludes a lot of cars.
Second issue is no one you see in car mags makes even a SBC header for a full size. That header is designed to fit a Camaro, Nova, Chevelle, Monte Carlo and the full size. It is a universal header You compare he part number with the cars they claim it will fit), but that means it universally doesn't fit any of these car bodies with out beating the tubes.
I gave up on buying an off the shelf header and instead buy a header kit.
Mild Steel kit
There are a couple of comapnies that offer header kits in stainless so it will last the life of the car body. I used this company to build a set of stainless steel headers that fit my car and engine combination:
Now I can weld structural steel, but I could never be a surgeon (which disappointed my father who started med school at Eaton, but WWII got in the way). For thin metal that has to be welded with inert gas inside and out I went to a muffler shop that does this every day. It cost me $1,300 to get a custom made header and dual exhaust out to the back bumper. Compare this with a Lemons header that is also custom made to fit your car, but doesn't include tail pipes or mufflers. If he doesn't have a previous build of your car body and motor (he probably does as it is a fairly common build), be prepared to ship your car to California and back to get him to build a set of tubes for you.
Or you can go to any muffler shop and talk to the owner/manager to build your custom made headers for you.