71 340 head choice (budget)

If the 78' Heads are in decent shape, they would be just fine for your build.

On your pistons, do they stick up above the head deck surface? If so you need to measure and get the proper thickness gasket if you going to look at closed chamber heads like LA-X, Edelbrocks (the non-340 hogged out ones) or Magnum EQ's. Regardless, find out even if you run the open stock style to get your quench right.

For that cam, I think the Performer or stock would be a better choice of intake, or step up in cams to use that Air Gap.