I'd start by checking with the local NAPA store, and ask for the oldest guy in the place, the one who still knows how to read a paper catalog. GM starter brushes and bushings were pretty universal from one car line to another- - - -at least up until the heavy duty commercial stuff got involved. Some of the online resto-ripoff places would probably be my second choice. 6 volt starters had a larger diameter armature shaft- - - -5/8" on the drive gear end VS 1/2" for 12 volt starters. I'm not sure if the rear bushing would be different size- - - -measure it with a dial caliper or a micrometer.

Even a little bushing wear can allow the armature to drag on the field coil pole shoes, so inspect the armature and pole shoes closely for drag marks.

Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!