[JM 35 Sedan]

This sounds exactly like the combustion/compression leak that I had last year on my way to Auburn, Indiana, driving my '35 fordor sedan. After dealing with this problem while in Indiana for the EFV-8 Club's CNM, and then driving back home to Maryland, I had to stop to add about a gallon of coolant every 75-100 miles. We later found the problem to be caused by a blown head gasket at #5 cylinder. Coolent was getting into that cylinder on the down stroke, super heated on the compression stroke, and forced out between the head and deck of block at the front of left side head as steam. This was apparent by seeing an area on the block and head that had been steam cleaned to a bare cast iron surface.
After removing the head, cleaning the deck, and mating surface of head, we used a Best Co. composition head gasket and bolted everything back together. I've put over 100 miles on the car since then, and all is now doing well. No coolant is being pushed out of the radiator overflow, and the engine is running much cooler at idle and when driving at normal highway speeds.