The obvious conclusion is that Clint would have played Harmonica, LVC would have been Frank and Eli would have been Cheyenne. But.

In the real OUTW, Henry Fonda was famously cast against type and played Frank. Suppose Sergio had adopted the same strategy if he had signed the GBU stars for this film? In that event, Clint would have been Frank and, assuming Eli was Cheyenne, that means LVC as Harmonica would have avenged Angel Eyes by shooting Clint in the showdown.

Turning this scenario upside down, who would have played who in GBU if Sergio had signed Charlie, Henry and Jason for that film? Well, Jason Robards with his marvellous penchant for playing garrulous types would be a natural for Tuco and perhaps even better than Eli Wallach. As Henry Fonda was the Bad in OUTW, you might think he would be Angel Eyes with Charlie as Il Buono. But there was no casting against type in GBU, so surely Henry would be the Good and Charlie the Bad? Charlie did, of course, sometimes play villainous characters, such as Kintpuash (Captain Jack) in "Drum Beat".