97 Jimmy - Broken Thermostat Housing Bolt

So, I was planning on doing a quick thermostat replacement on my 97 Jimmy, and seem to have screwed myself pretty good. The nut looks to be almost welded to the bolt, so I shot it with everything that I have to try to loosen it up. I was only using an open end wrench, and I'm not a great big body builder, but the bolt seems to have broken off in the housing somewhere. I haven't pulled it all the way out, as it seems to be threaded into the housing apparently. I haven't tried the other bolt, because I don't want to totally ruin my day.

What are my options for that bolt? I can't, by some strange twist of fate, get the other half out of the bottom of the housing can I? Am I going to have to drill and tap it and try to get another bolt in there? I don't really want to spend $500 to replace a $9 thermostat.... :banghead:

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