Is the only thing different with the v-6 t-5 and the v-8 t-5 the bell housing?

 

The difference isn't in the bell housing. A V6 T5 has a 14 spline input shaft, and a V8 trans has a 26 spline input shaft. Also V6 tranny's had lower gear ratios than the V8 tans.

You can get a special 14 spline clutch disk that allows you to use the V6 trans behind a V8, but if you are going to buy a trans for your car, try to get a V8 version and save yourself the cost of buying a new clutch disk.