1971 Demon with some coolant issues

If you can't find or have a hand pressure tester, they have these pills you put in the cooling system that turn the coolant different colors depending if it is leaking from either oil system or combustion chamber. I myself would change the oil and refill the coolant and change the thermostat. Run if for a while to get everything set again. If a motor sits for a while natural condensation from sitting with a valve open let's in a little water. Add that up. Same thing happens from air getting in through the breather, so it collects up in the valve covers also. After running it for a while pull the oil filter and cut it open and check for anything odd. Go from there.