The engine was rubber mounted on the GPz and later bikes, and both the crankcase and frame mounts are different. I briefly contemplated rubber mounting my 750 engine but after realizing how much work would be involved, I gave up.
BTW, crankcase halves should always be kept together as an assembly because they are line bored together. There is some president for swapping halves, but it's not an ideal situation.
I put a 750GPZ R1 engine in my '77 KZ650B frame. Just without the rubber mountings. The case was not changed, just the frame was changed for the rubber mounting. Yes. cases are machined together and need to stay mated for bearings can work and cases wont leak.
I don't have a plan and I'm sticken to it! '77 KZ750 custom project in progress
GPZ750R1 is a KZ motor a GPz750 is a ZX750 motor there not the same thing,
1980 KZ650 F1
Wiseco 810cc kit.
Zukiworks racing ported head.
VM 29 smooth bore's.
Dyna 2000 Ign. w/Dyna mini coil's
APE cylinder stud's and nut's.
APE valve spring's.
APE Track King clutch.
V/H KZ1000 sidewinder.
3.5x18 laced to a KZ1000 disk hub.
150/60/18 Shinko 006 Podium.
For what it's worth...apparently I was wrong with my comment earlier in regard to the crankcase halves being different to accommodate rubber mounts. Martin pointed this out in a different thread; the KZ750 and L3 model 750's (which used rubber mounts) share the same part number crankcases. The frame is different for sure, to allow for the different mounts, but you may very well be able to fit a later engine that was previously rubber mounted.