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Using Babbitt bearings, there is an additional defense against rear main leakage that divides rebuilders/restorers into two camps, pro and con. An excellent article appeared in a past MAFCA or MARC issue (I can't recall which at the moment) about carving a groove in the rear man cap towards the back and drilling a hole through the Babbitt into the drain passageway. What this last line of defense does is funnel oil on its way out of the rear main bearing area into the cap's internal oil passageway so that it can flow back into the pan via the drain pipe. I don't recall the exact place to carve the groove or the size of the hole (which is chamfered), but I'm sure someone can chime in with a link to the article.
I don't see any disadvantage of using this method. The amount of Babbitt material carved out is minimal. Most main bearings have spiral grooves carved into them anyway. What's one more shallow one? I like the idea of funneling soon-to-leak oil back through the cap into the pan. Unless I have missed some authoritative post-article criticism of this method, I like the theoretical concept. I did this to a friend's perennial leaker last summer and have heard zero complaints from him. And he ALWAYS complains if something isn't right in his opinion. So, it must have helped stem the flow of leaking rear main oil.
Another thing to do is to use a Chevy 350 drain pipe in the rear main cap. This larger diameter pipe will help flow more oil back into the pan, thus helping to reduce leakage.
Marshall