transmission help

[intermilanrox]

So after a couple months of driving around my 300zx without any reverse, my new transmission arrived. My cousin was installing it this weekend only to find out the bellhousing was different. So he swapped over the bellhousing, no problems there. However, when he went to connect the driveshaft to the output shaft the driveshaft wont fit. The new output shaft is 4mm smaller in diameter than the old one but still has the same number of splines, the driveshaft just simply wont get further than about 1/4 on the output shaft.

I've left a message at the place where I got the transmission (AmericanCarz out in Arizona) but I was wondering if anyone on here has any suggestions. The only thing I can think of is, the output shaft is wrong and I need to have a new transmission shipped out to me, or the driveshaft is old and needs to be replaced.

The car is a 1995 300zx 2+2 with an Automatic Transmission and just under 175k miles

[car nut]

What car did the new transmission come off of? Sounds like you'll need to replace the output shaft regardless.