Well, I've synced the laserdisc original English mono audio to the Mondo Italian Blu-ray but it does have its fair share of problems.
The Italian blu-ray is the best looking video transfer available but suffers from some frame jumping (missing frames) around the edit points. Apart from the occasional visual distraction, once you start noticing the jumps, a significant number of edits to the audio were required to maintain sync.
All dialogue was dubbed post-production and the lipsync is mostly pretty good but it's never going to look perfect.
The Italian cut is also longer, so audio for the missing scenes can either be taken from the Italian track or the 2002 English 5.1 audio where the actors dubbed English dialogue that didn't exist in the original theatrical cut. I chose to use the 2002 English track.
A better solution would be to edit out the 15 mins of additional scenes and get as close as possible to the English theatrical cut. That would be relatively easy except for Tuco's torture scene which is slightly rearranged in the Italian version. It's arguably a better paced film in the shorter version anyway.