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

A properly designed modern Viton/Silicon seal has to go in the right way. It has a front & back. It is designed this way so that crankcase pressure [ blowby ] puts some force on the seal lip to seal it.

If the slant crank has a knurled seal area, I would have that machined down if using a Viton/Silicon seal.