[rotorwrench]

It could be a head gasket leak. These old engines also like to crack in all sorts of places due to freeze up or overheating. You might check the spark plugs and look for clean ones or wet ones. They sometimes crack in the valve pockets so coolant could go right in the exhaust from there. I've even seen them crack inside the center exhaust port where coolant will just poor out the exhaust port. If you can nail it down to a specific side, you can just pull the head on that side and check it but I'd pull both heads anyway just to use both of the new gaskets that come in the top end set.