I have done this exact thing and in fact a VG30E meant for a 90 300ZX resides in my orginally equipped VG30i engine bay... they ONLY thing ... and I mean only thing ... to worry about is weather the crank shaft is a SHORT or a LONG ... either way is no real big deal ... all you have to do is look at the shaft, compare it to the other, and you will know ...
your stock engine should have a short shaft ... if you find a motor with a long shaft, you'll have to get the correct balancer for it (Nissan Part number -12303-12G01 - 189.00 dollars ) and everything will line up perfectly ... do not use the original harmonic balancer/pulley that came with it unless it is from a Pathfinder or pickup ... the car ones are different ...
Everything else will easily convert over.... (manifold(s), power steering, a/c, etc) if you get the motor from a 300ZX like i did, you'll also have to change the oil pan and pickup over .... trust me on this ... even with my TM 4" lift there was only about 1mm of space ... and do NOT ... I repeat - do not use silicon for a gaskget ... only use the rubberized Nissan piece ... you'll end up doing it all over again if you use anything else ...
plan on getting a new clutch set w/ throw out bearing too ... unless you have an automatic, this will save you tons of frustration in the future ...
I know the exact thing you are going through right now ... I stripped the key and key keeper completely and was the reason I chose to do the engine swap ... you can get by for a while by welding it like I did, but it will be out of balance, and will likely eventually fail ... replaceing the crank is WAY to involved for a novice mechanic like myself, and time consuming for a good mechanic ... do the swap and you will be a lot happier ...