And that's when things went south. The bolt broke off flush with the water pump. So now I had to move to remove the water pump. I drained out what was left of the coolant and and went to remove the other three bolts.... and broke another. Two did come out, so there's that. When I tried to get an extractor on the broken bolts, one did come out it was the other one that decided to break again flush with the block.
Queue ordering reverse drill bits and a new water pump gasket. This lead to removing the grill and radiator from the Jeep. Fortunately, these old military rigs were built to be field serviced, and that only took about 20 minutes.
I centre punched the broken bolt and started with the smallest bit I had, slowly working up to 1/4". Once there, the remains were still firmly embedded in the threads. I managed to use a small punch to pull up an edge. With that done I moved up to regular bits (1/4" reverse was my largest) and managed to get the remains to release. Once that was done I was able to run a tap through.