273 swap to 318/ build, cost, potential, daily driver

I'll add that since you're going to be building an engine, and I assume you will probably be putting in new pistons, I would worry too much about the head gasket bore size. Just get with your machine shop on how much compression you want, and get pistons that accomplish that given the head combustion size and head gasket to be used. It doesn't take much to CC the chambers to get some numbers to work with. Since you're not trying to wring out every scrap of horsepower, it's really a non issue for a street cruiser.