I grew up with English dubs, so I don't mind them, even if they usually aren't the best
Giuliano Gemma is from Rome, but he doesn't have a Roman accent; Romans speak very loud and with a lot of phrase modulation.
Gemma has a rather low voice and speaks in a more monotonous way (so without much phrase modulation, but his Italian is always clear
The Italian used in spaghetti western, was a rather classic Italian; modern words and expressions were omitted and usually "Voi" (comparable to the French 'Vous') was used as the formal form of address, instead of the today more common "Lei"
Like Gemma Franco Nero's Italian is clear and very understandable, but being from a more Northern region, he pronounces some sounds (especially consonants) in a slightly different, harsher way
Gianni Garko attended Drama school and has indeed a more lofty, more theatrical way of speaking his lines; it makes a very nice contrast with his laconic way of acting. He was born in Croatia but I suppose he went to Italy when he was very young, since he speaks the language without any discernable accent. His Italian sounds more like Nero's than like Gemma's.
like I pointed out in my TEPEPA REVIEW (is this a commercial break?) Milian refused to be dubbed for the first time for that film; he thought he had learned by that time enough Italian to speak his own lines. His idioms became known as 'Spitaliano', a funny combination of Spanish, Italian and fantasy language that contributed a lot to his popularity among students and (later) housewives. One of his best lines was:
Li mortacci tutti! which was supposed to mean 'Let's kill them all'
Italian has two verbs for to kill (you'll know them from film titles), 'Ammazzare' and 'Uccidere', but the verb Milian created, 'Mortare' doesn't exist (in English mortare would be something like 'to dead', meaning to kill). Nearly everything in his sentence didn't either exist or was in the wrong place, still everybody knew what he meant. Li mortacci tutti would be in English something like "Let 'm all dead!"
I don't know who did Sancho's voice in English, or any other dub
He was Spanish, so I suppose he did his own lines on the Spanish soundtrack
I have a few Spanish discs, but they also have Italian audio, and I always preferred it since my Spanish isn't great
But I'll check his voice and compare it to English and Italian tracks.