Allis b "soft seizes" but not overheating?

I would bet you have corrosion  on outsides of piston that takes up wall clearance. You said it "was" stuck. Unstuck and clean are two different things. Pull the head again and knock the pistons out to ensure all is well. Bet you find your answer.

Remove the thermostat, put in water in a pot on your kitchen stove. Turn the burner on high and use a meat thermometer to note at what water temperature the thermostat starts to open. It should start to open at the rated temperature of the thermostat. If it opens substantially late, its junk. A good radiator and cooling system flush would also be a good idea.

Wow. Thank you everybody, all this is most helpful. I guess I'm tearing into her again, hopefully for the last time! By the way, I've ordered a new thermostat but in the meantime I've been running with the old thermostat removed entirely and I'm surprised I can't see much of any movement of antifreeze in the neck of the radiator. But both upper and lower hoses get warm. The gage never gets over 120 degrees... so it makes sense the coolant is not flowing freely. Bill

I might try something like this:  remove the carburetor or manifold to gain clear access to the block's coolant drain hole, which I think is 1/4" pipe thread (an actual 1/2" diameter hole). Remove the petcock from the block to get a large of hole as possible. Fabricate a 1/4" copper tube or a 1/4" steel brake line say about 6 inches long to fit into the hole in the side of the block. Bend the tube to go around the corner to try and aim towards the front and back of the block. Connect this tube to a pressure washer and start in, flooding the lower block area for a short time and let it empty itself. I don't think faucet water pressure would be adequate enough to bust up the rust, but maybe. When the color of the water becomes pretty clear, call it good. 

Edited by DrAllis - 5 hours 33 minutes ago at 9:38am