318 piston questions

That being the case, I would suggest letting the machine shop use the pistons they have. That is as long as they are not just plain cast aluminum. For that $140 price, they had better be Hypereutectic. If you want higher compression later on, you could always get a set of 302 heads from a mid 80's 318. They have a smaller combustion chamber than your original heads.

What are you planning to use for the exhaust?