Sorry for the delay - I lost my post due to lightning-caused electricity loss (oddly, only in the wi-fi router room).
Most of my post got scrambled

Get a Best Gasket 6100 rear main seal.
"each kit includes installation instructions, a special knife, and a few other trimming aids."

Read this Tech Tip on Rear Main Seal Replacement.
Installation procedure on oldGMCtrucks.com - different gasket
Another article that mentions the Best Gasket Seal,

Maybe someone here can also find a good post describing installation?