any benefit to swapping heads?

Yup heads aren't an issue I'd leave them be. The best overall performance increase for the $$ would be to swap the cam to something with less duration that's designed for low-compression engines, and have the engine properly tuned. Actual CR is always lower than advertised on old Mopar engines anyway, from roughly .3-.5 meaning your "8.5:1" 340 is more like 8.0:1, unless it was rebuilt and blueprinted to factory compression of course...

Smogger big blocks (400, 440) rated at 8.2:1 come out to 7.8-ish, I measured the 1972 440 I overhauled for my cousin's '71 Satellite project and even with .010" milled off the head and the super-thin steel shim head gaskets the true compression came out to 8.0:1 even. Might be thirsty but it'll definitely run on regular, hell probably 80 octane lol