Professionl mechnic fixed rear main. Did it a number of times on serveral vehicles. Just did a 396 69 Chevelle, and it still leaks, can't figure what I did wrong. Any suggestion? Diffently coming from rear main. All oil plugs on back of block look fine. Pan gasket was ok.Sealed mating surfaces. Old seal looked fine

How long has it been since the engine was balanced?

maybe a possibility of a small burr on the crank? Could be ripping through the seal.

What type of seal was used? If you replaced the rope seal with a neoprene type, you may have installed it backwards. The open end of the chevron must face the crankcase. Also, the seal halves should be installed so that ends of the two halves do not line up with the parting surfaces of the block and main cap.