I've got three different Chevrolet flywheels on hand and one on my truck and all of them are 6 bolt flywheels. At least one of them is a 139 tooth flywheel which is for use with a 6 volt starter. The others are 168 tooth flywheels and they are for use with 12 volt starters. The crank on my old 52 model 216 was also set up for a 6 bolt flywheel.
6 volt and 12 volt starters are kind of hard to tell apart when looking at the teeth, but they are definitely different.
GMC may be, and probably is, a different situation, especially the earlier years, 30's and 40's.
Joe
