A 393 is a 351 bored .030 with a "3.85" inch stroke crank, a 408 is a 351 bored .030 with a "4" inch stroke crank. The only thing that will affect wall thickness is how far the block needs boring... if any at all.SLO95GT said:so, if i was going to eventually put a turbo on it, should i build a 393 or will a 408 hold up? meaning the thickness of the cylinder walls. i wanna put down some crazy boost. and what year block is the best. i heard it was the 1979s.