You don't happen to mean like this do you? Engine in the car and as I was torquing retainer pan studs,,,,instead of usual bolts,,,,felt a rapid loss of torque as though had stripped out aluminum retainer thread.

Milodon RR pan, ARP studs for main caps.

A bit of grinding with Dremel and all was well.

As an aside for OP, your rear pan bolts that go into retainer are required to be shorter than the other 18 pan bolts if you are using the slightly longer than stock pan bolts such as supplied in after market kits such as Mr. Gasket. They send you 20 bolts all the same length all slightly longer than stock,,,,,used at retainer they will bottom out before pan is tightly sealed giving appearance of a rear main leak.

1974 Motor Repair manual discusses this in section on replacing big block mopar oil pan. There also is a mopar factory directive that discusses this.

With engine in car this is not obvious, but with engine mounted on an engine stand at dyno this may not fool you. OP did say he had examined closely with a flashlight with engine running and that indication is that his issue is not pan leak however. This is how I discovered my problem with engine running in car. Mr. Gasket pan bolts bottoming were bottoming in retainer before pan was properly cinched up. Hence now, my use of studs.

Likely with OP as he has we'll investigated, not his issue,,,just a heads up for others who may be experiencing mysterious rear main leaks or what appear to be so.

I have posted a couple of other threads on this forum discussing my own journey and twilight zone adventure with confounding rear main oil leaks. .

Hopefully the peanut gallery will contain themselves and refrain from sniping at this post,,,,a private request that a couple of forum gentlemen might understand.