I'm pretty sure I found the solution that I'm going to share below in these forums, but I can't locate that thread so I'll start a new one to pass this along.

I had a problem with a 400ci small block chevy running the temp gauge to 260 before the thermostat opened. I also found that the radiator was overflowing regularly even after the temp dropped to norma. I'm sure the problem was an air bubble, but didn't want to re-open the system to try to fix it before exhausting all other means. I also didn't want to buy and install an overflow tank if I could avoid it. This is in a hiboy roadster, and I don't like the cleaner looks without the tank.

After trying every suggestion short of opening the system, I finally gave in and pulled the thermostat, then tested it to confirm that it was opening at 185 degrees. While I had the thermostat out, I drilled an 1/8" bypass hole as suggested in the forum I can't now find. I suspect that I purged the air when I removed the thermostat, but I believe that 1/8" hole really helps, too: the engine now warms up smoothly to 185 and holds there. It also doesn't puke coolant any more, so there's no need for an overflow tank.