Ashe, don't listen to these "specialists". I know of a few as well. Sure, they know how to restore the Z to original and how to tweak a set of Webers, etc. maybe even turbo the L6 to incredible levels. The issue is they are purists and they just don't want to see another Z go to the V8 or any other engine that isn't made by Nissan, IMO, of course.
There are PLENTY of 300, 400, and 500+ horsepower V8 (as well as other engines) powered Zs out there that live quite well with the R200 diff. If they mean the R180 that came in the 240, then sure, I agree, but the R180 to R200 diff swap is easy, cheap, and popular.
Welcome, we will not tell you things can't be done here unless there is a real technical reason, and then we will suggest ways to do something differently so as to hopefully meet your goals.