I'd start with 64-66 exhaust manifolds, run the HP single exhaust with straight thru muffler and resonator. Dual exhaust will gain a bit at the top end. At least you know it will fit and will still make plenty of power if you did everything right.
There is a set of manifolds on CL for 100, are these worth picking up? I know theres a bunch of them at the junkyard and i plan on swinging by tomorrow to have a look. Any particular casting numbers i should be looking for?