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

Keep in mind, the Slant-6 (like most other engines of its day) was not well known for keeping the oil on the inside. They could've done a lot better job designing that oil pan gasket arrangement, for example. But no, the rear main seal will not necessarily leak.

The rear main seal halves have to be installed right way round, not inside-out. The halves of the seal should not be quite lined up with the halves of the metal where they sit (the split in the circle of the rear main seal should not align with the split in the circle of the rear main seal housing). There should be a small dab of suitably good RTV on the seal-half ends to seal the halves together, and another small dab on the corner pieces on the lower rear main seal housing.

ETA: I really like the look of that 1-piece seal RRR just posted. That has to be rightside-in, too, and I'd put the installation split at 12:00 with a small dab of RTV.