Couple of things I have learned through the years.
First, either your heater is partially plugged, the heat control is blocked, or your thermostat is stuck open. Check the stat first by opening the radiator. If the engine's cold and coolant is moving, the stat is stuck open.
If you replace your stat, it is not recommended to go over 180* on the 235. The head doesn't like the extra heat and the higher temp makes it prone to cracking and warpage. Learned that after taking my head in the last time, machinist told me to go with 160* stat.
The 61 had a far better heater than the AD series, and would keep the cab warm below zero. Another thing to check is air leaks in the cab.
Cardboard in front of the radiator will usually help, but keep a close eye on the temp gauge on warm days.
