I didn't have to change oil pump shaft just bolted pump to cap. No issues
While possible^^, you need to check shaft clearance with the dizzy installed. I am thinking the adapting shaft is .135 shorter, because a std length shaft and a BBC pump can have negative clearance and actually push up on the dizzy and cause the dizzy to lock up and strip the gear to the cam. Been there and done that, so be sure to check it has slop when installed (fully assembled and oil pan off to be able to see if oil pump shaft can move up and down and isn't tight/jammed).
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