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I don't have the numbers on paper but it is easy to figure out. Get an U nrake in hand and with the rim being used mock up the arrangement. Unlike canti and V brakes the U brakes have their arms inter lace with each other. This makes the boss to boss dimension less variable as the two arm's ends need to relate to each other in a manor that other braze on boss brakes don't.
Another method is to measure a number of bikes with said brakes. There are thousands of BMX bikes and 1990s ATB bikes out there with U brakes. Since we don't know where the OP is we can't advice how close to him (her) these bikes might be found.
Lastly these brakes are still in production and some manufacturers might have some dimensional guide lines in their tec docs. Pauls and Shimano come to mind. Andy.