Hard to do wrong with the seal itself on the SBC but some other places of intrest in the area that can be mistaken at a glance for a seal leak.
Where the rear main cap meets the block there should be no burrs or "bruses" or the oil will find it's way through and appear at the main seal area.The factory started sometime to apply sealer in this joint--telling me they had a significent amount of trouble there as they don't spend a dime for sealer unless it benefits them.
Also, before 90's the obsession of the builder with thinner and lighter gave us some sheet metal covers and pans poorly designed and so thin the sealing surface could be rendered unusable by less than careful removal.I have been bitten by this booger and gave a proper cussing for my own better feeling.
I think this is your most likely source of leak.
Block up the vehicle and run it withyou underneath to view the area after cleaning up the wet area with spray brake cleaner.
Good luck
