I had the same problem as you but on my 400 chev. It turned out to be the rear main seal too. The only problem with it was the engine builder put in the wrong seal-it should have been a align hone rear main seal. Was your Chev 350 align honed? maybe that's it. This seal is just a little bigger. part #720-1962 from Jegs
I also thought it was the rear of my oil pan because it was chrome so I got the one piece fel pro "rubber like gasket" it's reusable and guarateed not to leak. Rear main seal leaks suck. I pulled my engine twice because of it. I now know that you don't have to do that. I just removed my motor mount bolts and jacked up the engine so I had just enough room to remove the oil pan. Sure beats pulling it out completley. hope this helps.