I had a leak at the rear main seal area right after I got the 383 back together in '03. I yanked the oil pan and took a look at it and the area where the saddle the rear main seal sits in and the block was where the leak was coming from. After cleaning the area on both sides of the saddle (left and right) with some spray carb cleaner (I used Wal-Mart - use that or Gumout but don't use STP) and gooping it up with some Ultra Blue silicone sealer, then reinstalling the pan and windage tray the leak was stopped. Make sure to use enough silicone so that there is enough to make contact with the pan gasket when the pan goes back on. Since I am using the factory windage tray I made a gasket and silicone sandwich: silicone on all of the pan mating surface, gasket, silicone on the gasket, then set the windage tray on that, silicone on the windage tray, then place the gasket, and finally, silicone on the bottom of the block on the gasket mating surface. Bolt it all up nice and snug and smear the silicone that squeezes out all along the edge of the sandwich. Let sit 24 hours and you should be leak free unless the rear main seal itself is your problem. Mine hasn't leaked since I did all this.
