Bill -
I think you are on the right course. I put in a new thermostat, refilled with coolant, raised the front end and after about 20 minutes of running got a few decent size bubbles coming out and heat in the cab raised into the high 130' coming out of the vents.

The lower radiator hose was still cold so I put cardboard in front of the radiator to cut down the airflow and it is now warm, so I think you are right about the coupling. Given it is pretty cold here the next few days, I think I may remove the fan all together and see what that does for me. My commute is nearly all highway so I shouldn't run into the issue of it overheating in traffic.

I did have steam coming from the filler hole, so heat was there. I think we've ruled out the actual coolant system. So the fan will be next.

Thanks for your help.