Before you worry about leaks from the rear main or sump gasket, check out the rear top of the engine and make sure you aren't getting any leaks from the camshaft position and oil pressure sensors.
You can just see them if you look between the firewall and the block at the top of the engine. Degrease the sensor area and the engine rear and bell housing using a spray can of degreaser, wash down with hose water spray (hose not a pressure spray), and see if the leak is coming from there over the next week or so. Common LS1 problem.