Interesting, I looked at this also. Going by the interchangeability, the seal groove in the block and cap should have the same dimension as the Pontiac I'm assuming.
If the assumption is correct, the only variable is the GMC V6 journal diameter vs the Pontiac journal.
To get a dimension for the sealing journal on a Pontiac I found this:
https://www.tinindianperformance.com...ontiac-engine/
3 inch main journal: 3.188" inch sealing journal
3.25 inch main journal: 3.436" inch sealing journal
Can anyone measure the crank seal journal diameter of the GMC V6? Don't have one apart to check right now.