After 14 years since the last thermostat change, the thermostat housing on my '74 Nova was leaking coolant. The mounting bolts were corroded and completely fused to the block and, inevitably, I broke off one of them. I successfully drilled out the remaining stud using a QuickCenter, but went too far with my 5/16" drill bit and accidentally poked a ~5/64" hole in the bottom of the mounting bolt cavity with the tip of the drill bit. I'm thinking I need to somehow plug this hole before reinstalling the thermostat housing, to prevent coolant from entering the bolt cavity. Any thoughts or advice would be greatly appreciated.