I have a 250 6 cyl. in a 66 chevy. Rebuilt the engine, keep getting water in my oil. Head has been completely reworked, milled etc. Top of block appears to be ok. Concluded, must be a small crack or maybe a pin hole in one of the cylinder walls. Looking to upgrade to bigger engine in near future. For now bought some K&W Block Sealer to solve the problem. The instructions say to put it in the radiator and run the engine for 20-30 min. Then drain the cooling system and let dry for 24 hours. Does that mean the block is to be drained also, and if so where is the drain plug on that engine.