Just about any motor will fit in a Z but they are all just about too much trouble. As with any engine swap you need motor mounts, tranny crossmemeber, drafshaft splice, and a bunch of little things. The RB swap has been done, but not by anyone on this board. I heard something about using 200ZR(300ZX with a turbo RB20) motor mounts, not sure what it was though.
Hmmmm a US LHD 300ZX with a I-6 wonder if its been done
Justin T