Spider gear press
Slip one spider gear assembly into place in the hub.
From underneath, slide a 3/8" drill bit (or something similar) up through the gear bearings, to keep everything aligned w/the shaft hole. (the shaft is actually @ 0.385-0.387", but 3/8" is the closest standard drill that will fit)
Press the shaft down through the bearing assembly, the drill bit will exit the bottom hole as the shaft advances.
Depending on what you have for a press, you may have to use a 3/8" stub between press ram & the end of the shaft, to get it to clear the syncro dogs (I did here).
BE SURE that the shaft doesn't go too far through the hub as warned of before.