GMC went to 6 bolt beginning with the '54 248. They and '55 1st series flywheels are 139 tooth and use a 6V starter. If you put a later 248, 270 or 302 in a 6V truck you'll need to find one of these flywheels or convert to 12V because they have 168 tooth flywheels.

Never heard of a 153 tooth flywheel. Are you sure it's GM?

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.