318 or 273?

The biggest one I ever saw was a 286. The block was wet sleeved to take the bore out to 3.7" and it was stroked to 4.44". Based on that, I don't think a 318 is even possible and a 273 would be horribly expensive. Probably the best bang for the buck would be the 198 rod, 2.2 Turbo piston, 225 that will give ~240 cubes, and they ain't cheap. Your best bet for those kind of cubes in an affordable package would be a V8...