Will my Bravo 1 gears work in a Sportsmaster Case?

I believe that the XR lower gears will go in the Sportmaster lower, not sure on the reg Bravo lower gears.

Are you asking about the gearcase only or a complete lower? Sportmaster has a bigger propshaft, other than that the gears will fit the case. So if you are changing all the guts from your standard drive no problem.

If you are trying to install B1 gears in the lower with an XR shaft, then the shaft needs to be cut down at the splines that go into the driven gear. The splines are the same size, just that the shaft diameter enlarges at the end of the splines. The XR lower driven gear ID is larger where the shaft fits in, but the splines are the same.
As Marc said, if you swap everything, no problems.
Hope that helps.
Dick