I just put a 350 in my 78 Chevy monte Carlo and thought the the rear main was leaking as well, but when i went yesterday to get new exhaust from the headers back they noticed that i was coming from the oil sensor on the back of the intake. That was what caused my leak . . .the oil was hitting the firewall and running down the side of the transmission (giving the apperance of a rear main seal leak) It's a quick fix, $ 7.00 at the local Advance Auto Parts Store. That's just my suggestion . . Good Luck.