To give my opinion/recommendation on swapping the shaft, I would not do it.
I stumbled onto this issue a few years back when I acquired a 58 johnson after having owned/run a number of 50s era RD models. I ran into it when trying to fit a 57 prop onto 58 shaft. Had to source a 58 prop nut.
You are more adventurous than I, as I would never have tried to swap shafts. Still, you (and we) learned something, and that ain’t all bad!
I’d keep the gear cases separate. They swap out so just keep the prop nuts with the appropriate case. Who knows, you may stumble into an earlier model RD/Bigtwin that needs that case. I stumble into them every time I go into my garage!😀
Happy motoring!
Scott
EDIT: I believe Dave is onto something there. Seems like many times with OMC, while it seems to us that a change was pointless, it made perfect sense from a maintenance or parts management standpoint.
