rear main on 383

Are you using the lip type seal?
Is it in properly? Are the surfaces of the seal housing and the cblock totally grease and oil free? Any oil -even from your skin - can cause a leak there. I use brake clean to soak everything down, then let it air dry or blow it clean. I use the side seals for the housing, an Ultra Grey sealant on the side seals and mating surfaces. The last leak I had was 6 years ago and i put 1/2 the seal in backwards...lol
You may also want to try the rope seal. It's not as easy, and it is harder to turn the crank when it's in place, but they won't leak. Butyou still have to get the other areas totally clean and dry.