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

This, again?

Bearing size (total bearing area) absolutely is the point; the crank is supported well more than adequately, and it's more than rigid enough for 4 vs 7 not to matter. Time has proven it; the Slant-6 bottom end routinely holds up for at least as long as the other-brand inline-6 engines marketed at the same time, and often longer—even the post-mid-'76 forged-crank ones with the smaller bearings. Even with extensive power upgrades.

This same baseless whinge—Eeeeeee, it only has four main bearings instead of seven, eeeeee!—was blathered in the 1960s by lazy, sloppy auto journalist Jan Norbye. He seemed never to miss an opportunity to cluck his tongue and scold the Slant-6 for having only four main bearings. He bìtсhed about this nonissue again and again and again. We see it here in this 1966 Popular Science article comparing the Valiant with the junk the other big three were peddling; lookit how he pretends to be astonished that the Slant-6 is so smooth and quiet despite having (tsk!) only (tsk!) four (tsk!) main bearings (tsk!):

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When he wrote this the first Slant-6s were only 6 years old, so their durability wasn't yet the matter of legend it is now, almost 60 years later. But I still don't give him a pass; he was given to spouting bozo opinions (“today’s drivers prefer warning lights rather than gauges”) and incorrect, made-up factoids ("The Torqueflite lacks water cooling"), then snappishly defending them when called out in letters to the editor saying he was full of beans. That's the pile of sewage this only-four-main-bearings-it's-a-wonder-the-Slant-6-runs-at-all idea belongs in.

Thiz another example of latching onto one little slice of theory, which might be perfectly valid, and making like it's the only factor in whatever is under discussion—ignoring other factors that can easily overwhelm and nullify the one latched onto.
I am thinking more of the overall strength of the engine. The 225 was rated at what 145 horse. It was designed and built with a strong enough bottom end to support that. I am surprised that any added power enhancements can live with the4 main block. Like I said, it must have a strong crankshaft. I am surprised Ma Mopar didn't build any 6 cylinders any bigger than the 225. That is undersized compare to the GM 230, 250, and 292 and the Fords 240,250, and 300 all which were 7 main engines. I come from John Deere country and none of their gas or diesel's would live any length of time if they were 4 main engines. Just not strong enough.