Is there any way to PERMAMENTLY eliminate rear main oil leaks?

Might also be leaking from the oil pan gasket. The two pan bolts there screw into the aluminum rear-seal holder and often get stripped. The shop should have verified those threads while the part was off and used a Helicoil repair if damaged. I bought the $35 silicone pan and valve cover gasket for the slant from Real Gaskets of TN. Don't overtorque them. I degreased all the bolt holes and used red thread-lock since the bolts are torqued so low, they might back-out otherwise. Best is a silicone gasket captured in a groove, like an O-ring so you bolt up tight w/ metal-metal contact, like some newish engines have. Someday, a clever Chinese company like Ningbo might make such cast-aluminum oil pans for classic engines.