The taper on a SB drum prevents themfrom sitting flush on the flange on a BB axle. We convert SB to BB here all the time using a fixture on a mill with a indexer. Threaded studs must be used.
Yep, That's what I do, however I don't have an "indexer", I have a drill fixture, with hardened drill bushings, that I fasten to the axle flange, and use a drill press. The fixture was made on a "jig bore" machine, when I worked as a machinest, about 30 years ago.