What degree Thermostat?

Make sure it's a high flow.
When I bought my Duster with a 360 engine, it only had a low flow 160. I replaced it with a high flow 180 and it suddenly got very happy (and the heater started working!). What folks don't realize is running the engine too cool and driving short distances can leave deposits on the engine parts and certainly make it more difficult to boil off moisture.