Headers vs. Manifolds

Also kinda depends on how much the manifolds will cost, and if they will fit. You wont know that till you get the engine in the car.
Personally, if I ever do a big block A-body swap (I have a 67 barracuda candidate, and a rb 511) I'll buy cheapo summit or jegs b-body headers, and "adjust" till they fit. I have a custom built set of headers for a bb/a-body, but they are semi fenderwell and too big, even for a 511.