Merry Christmas Jack,
The 7/8” shafts are used in diesel applications which are run at a low RPM resulting in a low load situation.
I couldn’t find any specs on SM shaft size.
ProComp (which I believe, is now SM) started to cast ford blower manifolds in addition to chevy blower manifolds, there were rumors that ProComp was going to cast SB Mopar blower manifolds but that never happened…..