'68 318 to carbed Magnum 5.2: Check my checklist

... Honestly, I'm starting to wonder if I would be best off throwing a pair of Magnum heads on it to bump up compression to 8.3:1 and leave the pistons and the rest of the bottom end alone. -Kurt

You posted this while I was replying, a set of magnum heads would improve the CR, and I think they flow better than stock but then you have to swap the intake too, unless you have the heads drilled for LA bolt pattern...plus correct head bolts, pushrods, rockers, etc...