[MrForbes]

The shaft is probably hard enough that it would need to be ground (or milled with a carbide cutter) to make the slots in it for pins.

Pins will act kind of like splines...the angle is better than the double D configuration. Perhaps spreading the load out among two pins, the double D, and fixing any end play problems, would be enough to make it work?

although the end play stuff gets more disconcerting the more I look at it, when I reassembled the transmission that I took apart for the exploded view this morning, it had somewhere around .030" play between the bearing and the snap ring on the shaft. There is a lot of slop there, the bearings are a press fit on the shaft, and it may be necessary to also press the inner bearing onto the shaft fully while supporting the outer bearing, and then install a selective washer between the inner bearing and the shaft snap ring.

This is kind of like working on car transmissions....which I've done a bit of.