Oh, by the way, I used the 28 year old stock 280Z trans mount to mount the 300ZX 5 speed to. It bolts right up (and that's the other reason why the engine is so far forward, although it's still behind the strut tower).
As for the shifting issues: I have a 350Z and can't shift the thing when it's cold out. The synchros for 1st, 2nd, 3rd, 5th, and 6th aren't worth anything when the transmission is cold. I got my transmission for dirt cheap, so I won't mind trying to weld the 300ZX bell housing flange to the 350Z trans. I have two spare 300ZX TT 5 speeds if it doesn't work out.