Thanks for the ideas guys, I'll check them out.Stever, I pulled the lower hose and filled the radiator while holding my hand over the lower opening. I filled it up completely then removed my hand and the water came gushing out very quickly, I did this a couple of times and the first two times there was brown sediment that came gushing out with the water. So the radiator doesn't seem very clogged, but maybe getting some of that sediment out will help.
I haven't yet removed checked the bypass passage to the water pump. My next test was going to be filling the radiator with the thermostat removed, and the heater passage blocked off, then run the car and see if it pumps water out. But I can't do that without a lower hose.
I also took the opportunity to back flush the engine block and it flowed clean water well.
This is slow progress, but I feel I have pretty much eliminated everything except the water pump impellar slipping or something with the bypass passage.