318 to stock 340 power level questions.

The ports on the 318 heads are smaller than those on the 340. A 360 head will have bigger ports but it also has a larger combustion chamber. This will reduce the compression ratio. The 68 318 already uses the tallest piston in the "stock" piston inventory. If you want to recover compression, I'd suggest an aftermarket piston rather than milling the head and manifolds.