Having tore down both 318 and 340 Ill say I agree. It's in the details. People say the 318 is identical, only a little and its externally. So in theory all the little parts you'd have to obtain and swap into a 318 make it expensive to build up. Anyone who knows knows this. For example you are going to buid up a 318 you need heads. OP has 360 heads ,great they are already one step ahead...but wait, there's more...the pistons, the cam...you going to rev up to 5500-6000 is the bottom end going to hold up...etc.? the heads need milled does the block need bored? how bout the crank? Machine work how much will it cost? What are the gains after the money is spent?