Did a quick search for the cork rear pan seal... found lots of threads where I link to the now dead "pontiac street performance" website that had an excellent write up from Mr P-body.
But I did find a post from Cliff that showed a couple pics of using the cork seal. Mr Pbody stressed using ultra black RTV liberally on the gasket to cap & gasket to pan as well as a bead on the ends where the side rail gasket meets the cork U seal.
https://forums.maxperformanceinc.com...+pan+cork+seal