pic: NanoShifter

[A_Reed]

Step 1: Press 3/8" bearing

Step 2: Insert small shifter gear into tube, align and insert shift shaft with dog assembled.

and

Brick wall..

even if you flipped the last bearing there is no way to get the other gear on the shaft. It may have some way out there solution but I don't see it. The hex boss on the shifter shaft creates a mechanical limit.

May I suggest putting a hole large enough for the gear (and entire shifter assembly) through and use 1/8" plate to patch the hole. To determine alignment use pins and threaded inserts for fastening in such a compact space. You may have to do this to insert all shafts and gears. Using the tube to determine spacing and fit, Swiss cheesing the outer wall and making a single face plate to close off the assembly.