As I see it, the issue here stems from the fact that you're not sure if the engine is one that was even available, and therefore if the exhaust manifolds drop in the same location. I kinda think you're likely OK with corresponding 300 head pipes... but you gotta be sure or buying a premade kit could be an expensive mistake A bit more research should answere those questions and allow you to make an informed decision.
If you're in a quandry about that, do what Snotty says. Unless you're attempting to recreate a close to stock resto (which you're not), a shop-built system made by a pro should fit perfectly - in my opinion well worth the extra cost and elimination of all fitment headaches.
Here's another option...
In my case (and because my car is all factory) I am getting a pro to build me a stainless system using what's known in the industry as "data bend cards" which tell the fabricator and machine exactly where to bend the pipes. Either the fabricator follows instructions or they have a really expensive state of the art machine which takes a straight pipe and spits out the bent pipe at the the finish. Either way it's a factory fit. In your case he could do that with your mid and tail pipes and hand bend the head pipes.
Data bend card are available right back to cars from the 30s from databases of major exhaust suppliers like Walker and Maremont etc who build original issue exhaust systems for the factory and then supply the aftermarket.
