What do you call the valve on the exhaust cross over?

It sounds like a heat riser valve. You might look for one on ebay but nobody seems to make reproduction ones.

You can try Green enterprises,they buy out new old ford parts stock,but the catch is you must have the OE ford part number. You can also try Dearborn classics

That is the heat riser. There are numerous places online that sell them or you can try an actual parts house not Auto Zone or anything like them. They normally only have common parts, but do try Napa or Car Quest.

It IS a Heat Riser valve but unfortunately you are not going to find one at Autozone. They don't even have parts for my '89 Crown Vic. Their computer only goes back to the early 90's. Pepboys goes back to 1968. Advance goes back to the 80's. Nobody goes back to the 50's except obsolete and reproduction suppliers but that's not a high demand part so no one is going to have a replacement. You'll have to have it welded open by any reputable welding shop as I doubt you will ever find a replacement. All that part does is helps the engine warm up faster in the winter time by closing the exhaust slightly and forcing hot exhaust air back into the exhaust manifold and the air rises up thru a piece of metal tubing to the choke on the carb giving some heat to the carburetor so the gasoline burns better while the engine is cold. When that spring is cold it closes the valve partially, keeping the heat in the engine and sending heat up to the carb to help atomize the fuel. as the spring heats up it opens the valve and lets more air escape thru the exhaust pipe. When the engine is completely warmed up, the valve will be completely open or atleast that's what they were supposed to do. I remember them on many cars and trucks back in the 50's, 60's and early 70's. They were always a real pain in the butt and never really worked the way they should. Unless you are driving it daily and in very cold weather which I'm sure you're not, you really don't need it and can have it welded so it's open all the time and disconnect that metal tube from it unless you want it to look original then leave the tube there but you can still weld the valve in the open position. Kenny.

He did not mention you can add a heater kit if it ices up. When it's humid and cold, but not that cold they ice up where I live somethin fierce. we had a few in the 80's that sat and had limited use.