Put it all back together, new t-stat (verified working in a pot of boiling water) with small hole drilled in top and it still behaves the same way. It didn't overheat, but it was staying around 220 with a temperature reading of 225 at the t-stat housing. Temperatures outside is around 75deg, so it's cooler today than most of the days it's overheated. I drove it for 3-4 miles and took readings at the radiator. The inlet was around 180-200 depending on where I took the reading and the outlet was around 160. There were still readings of between 100 and 130 on the front of the radiator that I could test.
Could a bad head gasket or cracked head prevent coolant from flowing through the radiator? Could there be a blockage in the engine or head somewhere that would cause it?
Not sure if there is much more that I can do other than pull the head, but I'd hate to do that just to see if that's the problem! I guess I see if I can find a decent mechanic to run some compression test.
I've been working on car and boat engines for almost 30 years and I've never thrown in the towel on a problem. This might be the first time!