My 71 Dart Swinger was the same way, I changed to a 195 thermostat. In Vermont It took a while but I got very little heat. I did some checking and put a thermocoupler in the radiator. It doesn't get above 170 or so, /6 is just to effiecient running. So I experimented, I took a piece of aluminum and drilled a small hole in the center instead of the thermostat. After a while I got it to run at 190+-10 degrees. I don't have it anymore so I can't remember the exact setup took a bit to get it right, in the spring I took it out and put the thermostat back in.
Hope this helps,
Rick
