what manifolds for magnum

Did you plug the smog ports on the heads?? 340 engines (manifolds) were not designed for smog ports so if you don't plug them it might be your problem.
From every thing I've read magnum manifolds won't work with P/S. At least I couldn't get them to work. The 340 manifolds should work, I have 302 castings on my '69 Barracuda and the magnum manifold (passenger's side) works fine. The 302s have the larger machined surface at the exhaust port just like the magnum heads but no smog port. I have a set of newer 360 heads to use on my 318 and I plugged the smog ports before I did anything else.

Treblig