It's not gonna be that easy. Apparently, TGTBATU has a couple bits in the American cuts that are not in the Italian one. When Eastwood takes his share of the gold, there's a shot of his horse rearing up, and then Clint says "Sorry, Tuco."
The Italian mono on the MGM disc actually has matching audio for these parts, but these two shots have apparently always been absent from the true Italian cut. Conversely, they have always been in the American cut, since every video release going back to the old pan-and-scanned videos has had them.
There are also differences in the scene of Tuco being tortured. There are some comparison clips of both in this thread:
http://www.fistful-of-leone.com/forums/index.php?action=printpage;topic=10097.0
If there are bits missing from the Italian cut, then conforming the English mono to it might be difficult. I'm thinking of what CatBus had to do for Monty Python and the Holy Grail.
Either that, or make a hybrid of the Mondo and MGM video sources to go with the hybrid English/Italian track.
Returning to an earlier topic: SilverWook, any luck on the Grease LD?
Also, I discovered that at least some foreign mixes of that film were different - Canal Hollywood in Spain shows what looks to be the restored version, but with an old, mono Castilian Spanish dub track. It matches the 1978 mix, but some sound effects and things seem to be mixed differently.
In "You're the One That I Want," the sound of Sandy grinding out her cigarette with her toe is really loud. You can also hear Danny throw away his sweater, though it's not as loud as it is in the remix, and I didn't hear the loud "thump" sounds when Danny's sweater and Sandy's jacket hit the ground, which are only in the remix.
Also, the vocals of Jan and Putzie, Sonny and Marty, and Doody singing along are mixed in a lot louder than the original English mix. They're basically at full volume. (This is also true for the modern, remix-derived German dub, but as with the previously mentioned backup vocals, they are horribly out of sync with the rest of the music...)
Some of the walla sounds kind of different in some of these dubs, for example, the wolf whistles when the made-over Sandy first walks in, but I'm wondering if some of those were actually unique to each language's dub...