This has several " options" as to what could be wrong. Its never clear without seeing it in person, but that being said, here are the common mistakes:

1) Seal in backwards
2) Seal retainer wasnt lined up with seal grroves and/or checked
3) Rough crank surface
4) Side seals not all the way down in the slots, OR , protruding from top of seal retainer. Overall mis-installation of side seals.
5) Spot facing needed where the bolt heads rest inside the cap, Errosion etc often leave the caps cockd because of wear there.Spot facing of bolt bosses does help.
6) Seal retainer ends need to be cut in a cap grinder to be squared and add a slight bit of tension to seal
7) Oil pan bolts too long and tighttned too far, cracking open the " wells" in the seal retainer, and oil leaks out the threads.
8) As mentioned, two different " planes" between seal retainer and pan rail where the retainer and block meet. Oftn a good gasket seals this up.

Many times a seal is to blame, but it can also be the side seals, the oil pan bolt threads, a misfitting retainer and also sometimes simply filling the groove ont he OUTSIDE of the seal retainer to the block will stop what is believe to be a bad rear main. I only use Superformance and Fel Pro 2904 seals. Stock ones will work fine, but never as good in my opinion as the Viton style. You also can have problems when a seal surface has been polished too much, or is too rough. Sometimes a slight polish helps, but its a good idea to cut the retainer to add some pressure afterwards. The old anti leak rear main bearings did work, but you really shouldnt need one if everything is correct. You can also switch to a rope seal, and often times it will resolve a problem, or at least band aid it. Again, if you know what to look for, and how to do it, rear mains dont and shouldnt be a troubling leak.

Long shot,there were some cranks, typically marine cranks that ran reverse rotation and had the seal serrations backwards! Doubt thats the case here, just saying.

Good luck.