I had assumed my rear main was leaking on my '79 350, changed it out but still had the leak. Turned out to be my Oil Sending Unit. The fitting wasn't set properly on installation and threaded wrong into the intake manifold. It just steadily dripped down the housing, which caused oil to drip from all the holes in the housing, and covered the starter bolts which made me think it was my rear main. Check your oil sending unit, my '91 Blazer has a P.O.S. plastic one on it, which is also leaking.

 

Darn I wish the bean counter and "engineers" had to work on these little monsters. Our 99 has the same problem and even to change the adapter O-ring and gasket looks like an all day knuckle buster. Bought the kit from NAPA it is a Fel-pro kit #ES 70016.
My brother-in-law just pulled the engine in his 99 and it was a about a 3 day affair. He's a retired professional mechanic and now hates GM and swears he'll never buy another one.
I tried the Blue Devil stop leak. It slowed down the leak for a couple of weeks but is back now bad as ever. Don't waste your money on that stuff.