TC
915 box is superceded the 901 box and is a 5 speed. A cost effective way to get a 5 speed, built to handle the horsepower and torque, but there are some installation problems. Shift pattern is "standard" and not dog leg like a 901.
Some years are more desirable than others, and this is what little I know - I am sure others more knowledgable than I can advise if you decide to go this route:
Post 78 have an electronic speedo.
Later models (86 +??) have the 110mm 930 size CV joints, earlier models use the 100mm per type 2 and 944. No big deal as the output flange can be changed but beware.
Nose cone can carry either 1st. or 5th gear depending on the year. Later ones with 5th in the nose is more desirable as 1st in the nose cannot take as much abuse.
5th in the nose however can burn up under long, sustained, high speeds. 83 - 87 models have an oil cooler dumping back over 5th to solve this problem.
LSD was an option and not all have it.
Like Cory says it is fatter, but the big installation problem relates to the increased length, shifter rod coming out the bottom, and clutch actuation is opposite to VW and push to release.
I have chosen to solve the length problem by cutting out the centre of the torsion tube, allowing the engine to be moved forward, and run coil-over rear shocks. There are other ways to solve this problem including a kit that allows a bolt in.
Bellhousing mounts will need to be fabricated.