There is a new ring gear listed on Amazon for Pontiac GMC that shows it as 176 tooth . They also have some of the dimensional specs . With that being said I would not put a lot of faith in the description of something found on Amazon without a second source of verification.
https://www.amazon.com/176-Tooth-Pon.../dp/B07MMHYFGX
For your pickup with original V8 there were only two flywheels made . One for a manual transmission and one for a hydramatic transmission . The pickups with manual will interchange 100 150 250 and 300 series . The hydramatic has a bigger interchange even into the bigger series .
However 350 370 400 410 450 and 500 had two different options as far as v8 manual flywheels . Possibly (not sure) but maybe larger because of being more heavy duty in that application torque etc.
Running of memory here but I think when you ordered a flywheel from GMC it was also paired with the correct housing . So pretty sure the pickup with V8 would have had a smaller housing that say a 450 series .
Sure you know this but 6 cylinder flywheels or housings do not interchange .
I hope not the case but someone may have paired a big truck flywheel with small pickup bellhousing. Thanks good luck