wow, you must be fast with that. Me, whenever I start taking the bolts out the thing starts leaking and a 2" hole?I agree that the thermostat cover probably is the source of your leak. It should be perfectly clean and smooth, since it mates right up against the cylinder head.
You can replace the thermostat without draining the coolant, though. I've done it several times. You just need to have your new thermostat ready, and act fast. You won't even drain the expansion tank if you're quick enough.
seems easier to drain about a gallon out of the system and just take your time.