Fixtures are a big help when building trannys. Here are some pics of some home made fixtures...
The case fixture has 2 bolts (left and right in the first photo) that line up with the cast bosses in the case that are intended holding points. The other 2 bolts are for leveling the case. There is a piece of pipe that can either be slid into an engine stand or clamped in a vise for holding the fixture.
The output shaft holding fixture bolts to transmission mount holes of the case and utilizes a drive shaft yoke to hold the output shaft. The piece of all thread lets you adjust the height of the output shaft in the case for assembly.