Make sure you are using a 180 thermostat. I used to have to block the rad with cardboard deep in winter in northern Indiana. The /6 has a large block area on the outside, so tends to run cold very easily.
I put a 195° thermostat in the Duster I had back in the day, though it had a 318, not a slant. Not so much for driveability issues, but to actually get something out of the heater in cold weather. My current Dart (with a slant) sleeps in the winter so it's not an issue.
Don't recall the Duster having any particular cold weather driveability issues, or at least any worse than anything else out there at the time. Cardboard over the radiator was pretty much SOP in the winter here in northern N.H.
