Big Chevys are bad about leaking between the main cap and the block. The machined surface can often have quite a few tool marks, and oil can find it's way by. Most builders place a small amount of sealant, such as Permatex, on the face to cure this.
I never liked using silicone there, as it will not displace like # 2 or #3 aviation Permatex.
Also, with the two piece seal, always put a dab of gasket making silicone on the face of the split, as it can leak there as well.
Of course, to do either of these, you have to drop the pan.