since my knowlage is more chevy based I would still suspect a head issue. its not like cast iron to grow shut. I have done hundreds of the old straight 6 chevys and the heads cracked. its their nature. I recomend pulling the head and having it checked for cracks. if none are found then I would put in a new head gasket and call it good.
its lttle effort now to repair then out in the trail.
secondly the tube chassie would be a huge step up as would and ELECTRIC fan. to take care of cooling.