Dan,
I have the recirculating heater and it is the Canadian version. I found out when I replaced the core that the Canadian version has five rows and the US version has three. I had to have mine modified to use the US style core.
Your truck should heat up to operating temperature in the summer, without a thermostat. That is how I started with mine, to make sure there were no overheating problems. Also from what you describe, it sounds like your engine may be starving a bit, possibly due to a carburetor problem.
My truck was commercial blue then I got it but that was pretty hard to tell because of the rust. You can see it in the gallery.
