The stand-alone remastered Blu-ray of The Good, The Bad and The Ugly from 2014, has the original English Mono.
The gun sounds are all original etc. It obviously downmixes for the extra scenes, but it’s original everywhere else.
Surprised not a lot of people realise.

Also the Italian Blu-ray of Giu La Testa (Duck You Sucker) has the original English Mono track as well, with the correct music in the final flashback, but has Italian Audio for the final line “What About Me?”. But is infinitely better than the censored audio on the MGM discs.