An update. Today I picked up a new gasket and took the thermostat housing back off. Problem is I underestimated my own stupidity. Apparently, when I was tightening the bolts before, the thermostat slipped down the housing just a tad and was allowing coolant to bypass it. There was just a tiny gap, maybe less than 1/8", but that was enough.
I took the housing completely off, marked the top where the little jiggle valve goes, cleaned it well, took it inside and "glued" the thermostat in place with the RTV then glued the gasket on top of that. I let it sit inside until it had fully cured, locking the thermostat in place. Once it had set, it wasn't going anywhere and I was able to install it without worrying about it falling out of place again.
Started it up, within 5 minutes of driving for the first time ever, the gauge was on the half way mark and I had glorious, glorious HEAT!
Apparently the design of this makes it easy to make this mistake as the old thermostat installed by a previous owner had the same problem. If you're having overcooling problems, this would be the first thing I'd check. Hope this helps somebody.
Thanks to everyone for your help and ideas.
Last edited by ben1
on Mon Dec 10, 2012 6:34 pm, edited 1 time in total.