Sunday drained out the coolant, put a jug of cleaner-deruster and filled with water. Ran for 20 minutes using the old radiator cap. The spare cap I have is a 25# racing cap so I figured I didn't want to put that on there. Pressure in the hoses was obviously higher than it probably should be. Cooling system never got over 200* Auxiliary fan never came on. Did observe some occasional bubbling in the overflow but attribute that to the cap issue. Surface temp on thermostat housing was 195*, radiator hoses were 165*, heater hoses were 135*. Heater output didn't seen to be as high as it should be given the coolant temp. Can't really check that with an infared thermometer though.

Next steps are to drain system and see what color comes out of it. New thermostat and cap will be added. While I'm at it, disconnecting the heater core and running some pressurized fresh water through is also on the agenda. I'd like to see if any more stop-leak blows out of it. Based on the heater output, I fear it may have a build up in it.