Long tube headers vs Block Hugger (short) Headers 273 Small Block

For your level of performance I wouldn't worry about headers. Your 75% size of a 360 and mild 360 don't need headers.

Headers are more important if you got plans for large overlap cam, high flow heads etc...

To me me 340 manifolds are over priced for the couple of hp advantage.
At the most just grab some 360 manifolds might pick up and hp or 2, Chrysler did the samething and added 360 manifolds to 318's in the 80's.