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I had that problem too. My U had a 403 and I was plowing in 4th gear. It could work O.K. all day in 3rd. I had too much engine or not enough radiator, depending on your point of view. By chance, have you removed the thermostat? A thermostat will slow down the speed your coolant goes through the system, because it has a smaller opening than anywhere else in the system. Of course, if you install a new thermostat, it needs a 1/8" bypass hole in it so that it will open as soon as the water in the block gets hot. Does it use a pressure cap, or is it the old style-like a gas cap? I'm not sure this affects your problem, but a pressure cap will allow the engine to run a little hotter with no cooling problems. How full are you filling the radiator? It takes about 5 gallons to fill a U and 5 gallons will expands right much from 80 degrees to 180 degrees, so if you have it close to brimfull cold its bound to lose some coolant when it warms up. There's also a chance your radiator is plugged up, or you have a lot of scale in the blocks and it is retarding cooling. These are just some ideas to get you started.
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