Other than the axle seals and input/output shaft seals there are no gaskets in the trans.

I would definitely pull it. Considering how much power he made, I would say there is some definite bearing wear and it wouldn't hurt to re-shim it, especially if you're switching diffs with different bearings and wear. The pre-loads is basically what keeps it together.

Use Permatex grey high torque sealant on all the case sections and clean every surface really well.