Once SunTour changed from the 2-prong to the 4-prong remover the chance of inflicting damage decreased. The Ultra 6 allowed you to have six speeds in the space of a conventional freewheel's five so, as suggested above, you might need to tweak the limit screws. I think the ProCompe used 6-speeds in the conventional (wider) spacing but if you don't have indexed shifting it should not matter..