You will require a 3/4" spacer (washers, big nut ETC ) with 12 mm holes in the spacers between the transmission mounts and the frame tubes otherwise the 4" longer 5 speed transmission tail section will hit the gas tank, make a hole, and go BOOM

the transmission "bell housing" side of the torque tube looks different, and the input shaft is a different length but it all works out OK

the one thing you have to do is to switch the shift shaft between the 2 transmissions, the 5 speed has the cut in the shift rod in a different position- if you try to make the shift linkage from the 5 speed, with the 4 speed shift rod, you won't be able to engage 1st, 3rd or 5th. it is simply a matter of unbolting it off the 4 speed, and putting it back on the 5 speed.

Option 1: (The way I did it)

Get your 944 transmission and the 924 transmission and put the input shaft piece from the 924 into the 944 transmission. The 924 input shaft is a different length and is 20 mm vs. the 944 transmission which is 25 mm. Also replace the shifter bar. This makes for a 10 second easy install back into the car as the shifter bar is not in the way and you don't have to angle the transmission as much!

Option 2: (More $$$)

Get your 944 transmission. Get a 25 mm driveshaft (also called a torque tube) off a 931, get a 931 Clutch & Flywheel, put them all together.