10.5:1 273 pistons with 360 heads compression ??

I'm not entirely sure what the height differences are between the two but I believe that's where it's made up because the hipo 273s have reliefs, the lopo don't. If I had to guess I'd say you'll gain 1/2 point with the pistons, but you're losing a full point with the heads. Instead of pistons, you might want to have a set of 273 bore thin head gaskets made up. They won't affect the balance, might be around the same price, and will get some of that compression back without too many side effects like additional milling.