For me the issue is the quality of the vehicle that you are starting with. My '73 is virtually all original and I will do everything I can to keep it that way. If on the other hand I found a straight, clean Z for sale with a blown or questionable drivetrain and lots of aftermarket stuff I would definately consider the swap. If I did mess with the motor I would probably go all the way and do lots of "other" upgrades. I can see it now:
A '73 convertible Z with a 327, 5spd, 17 or 18 inch wheels, dual exhaust, shaved handles, thumpin stereo. It would look and run just like the original Ferrari in Miami Vice!