The main
oil passage from the pump to
oil filter adapter should be opened up to 7/16", and the
oil feed holes in the mains are partially covered by the main bearings...mark where the holes should be by putting a main bearing in place and mark the area on the main saddle, then open the oil feed hole with a die grinder. It's also a good idea to place the oil filter adapter mounting gasket on the engine and mark the openings from the gasket onto the block where the oil flows to and from the block. You'll notice the oil has to make a 90* bend out of the block and back into it; round these "corners" in both the block and oil filter adapter (much like porting heads) for more efficient flow. Also, be aware that the acid in your fingers can damage a bearing.
Good Luck,
Dan