How Big can a 5.2 get?

Whew, well, if that’s where your at, figure that a 318 stroker kit is $*** vs what your doing. If the math works out OK for your wallet, a (near or actual drop in) stroker kit may not also be easier but favorable.

Going from 318 - 390 a 72 cube jump in what is going to be a huge torque jump doesn’t sound so bad.

From a cylinder head stand point, to feed that many cubes well is going to require very well ported heads.

Leaving the heads port stock is going to limit HP of course but will keep a lot of torque and probably really responsive on the throttle.

Projects like this can snowball badly sometimes.