It seems to me the wrench size on the old stovebolt engine starter was 3/4 in.
Recently took off a starter, and you are correct with the bolt/nut size.
Access to the top stud is a bugger, I don't know what GM was thinking at the time as one can only get about 1/16-1/8 turn with a 3/4 open end. I make sure the threads on the stud and nut are in good clean condition so that the nut can be finger tightened (or loosened) most of the way.
