Many of the big port 302 heads are around 120-ish CC's, so you may be able to trim and profile a considerable amount of dome off of them. I have spent many hours doing the same thing to raw "blank" dome racing pistons before. If the pistons are a "hollow dome" style you may be limited as to how much can be whittled off of them, depending how big the dome ends up being volume-wise. You'll need to CC a piston dome in one of the cylinders to calculate the compression once you CC a combustion chamber. You'll also need to know the piston deck height in the block as well to make your calculations.
