I was told by my engine builder, that they recommend using a 3" exhaust with thier header. Right now I'm running a 2 1/2 exhaust. Is there anyone who makes a pre-formed 3" exhaust for the 240Z? Or is this going to be custom. How much of a difference would 1/2 of an inch make in performance. Any recommendations on this would be great . I'm running a 3.0L F54 block, N42 head, bored SU's, cam with about .490 lift.

I don't have any suggestions towards your engine builder's recommendation of a 3" exhaust, but I can tell you that you're looking at a custom built exhaust. I do not know of anybody that has a 3" exhaust preformed to fit a Z. You should expect to dish out at least $250-$300, minimum.