Reply to "Clips too thick... ??...."

That has happened before, especially with aftermarket u-joints sourced from other cars, which may have ends 1 mm too long. In a pinch, sand the too-thick clips down on emery paper until they fit comfortably in the grooves provided. Much less work is to obtain thinner clips -possibly used- from a driveline shop or another vendor, some of whom include two thicknesses of clips. Some enterprising owners sand down the two slightly too-long ends on each u-joint. All these methods work if done carefully.
Good that you caught the problem early: a failed halfshaft at speed will do one of two things depending on whether its an inner or an outer u-joint. It will either beat up the pass-through area in the inner fender panel- possibly also taking out the rear brake line (outer joint), or it will hit- and crack- the aluminum ZF case (inner joint). Both have happened multiple times in all years of Panteras.