I think the correct L/R drum snap ring is angle cut on the ends. With the drum out the snap ring should fit in the groove. The correct thrust washer should have two anti rotation tabs on it.

There is nothing in the OD unit that would cause this issue. Only thing I can think of Is the ORC cam/rollers/race are not seated. The race should be pressed on the drum. Make sure it’s pressed on all the way. The cam should be pressed in to the case without any clearance between the cam and back of case.

As for the effort to seat the overdrive, that’s not uncommon. If you used the correct tool to assemeble the OD in the press and didn’t manhandle the unit/drop it before assembly on the trans then it’s just one of those units that’s difficult.

You have to keep the planetary and direct hub lined up during the OD build process.