Piston to valve clearance may dictate which camshaft you can run without notching the pistons. The RB STX15 is .249@.05". You may not have the clearance you desire if ground on 108 or tighter. FWIW, I have a 383 with pistons .02" positive deck. I was able to fit a 242/252@.05"/110 with milled 906s 2.08/1.74, but it was tight (.060") and there was no room to move the cam.

None of the heads are too big.