tstat housing

If the thermostat housing rusted thru, I would be concerned about everything else that touches coolant, especially aluminum - SB timing cover, radiator. Normal ethylene glycol coolant needs to be changed every 2 years or so, and if you don't run the engine often the anti-corrosion additives can settle out. I am thinking of switching to Evan's Waterless Coolant (propylene glycol) in my old Mopars. I did so in my 85 M-B already. It costs 4x more but lasts forever, no more corrosion, builds no pressure, and harmless.