Suppose that Clint, LVC and Wallach had never existed and Leone was supposed to cast 3 other actors from the available pool at the time to play the part. Also suppose that production had begun during '67 so that many SW actors had already established themselves. In this alternate universe, I would pick:
G: Anthony Steffen
B: Benito Steffanelli or Nello Pazzafini
U: Eduardo Fajardo or Frank Wolff
This would make the film vastly inferior to the real GBU, but given the circumstances, what else might Leone have done?