Good advice here.
And also mid last century when I was a motor mechanic we would also replace the spigot bearing if we were doing the clutch. Just good practice to replace all the bits in there while you are in there.
That's good advice too - but, there's no flywheel spigot bearing on the front-wheel-drive FIATs.
I think we can let you off because front-wheel-drive didn't become popular until the final twenty years of last century.
The relevant job to do instead is to check and possibly replace the input shaft bearing and seal on the gearbox, since that performs the task of centralising the clutch.
If the gearbox input shaft bearing is badly worn, that could cause the input shaft to waggle around but centralise itself when the clutch is disengaged. I find that fairly unlikely though - there would be the most horrendous growling/grinding noise from the gears and a bad oil leak. However, it would cause shaking from the out-of-centre clutch.
I wonder why all the mountings, subframe, and driveshafts were replaced - were they damaged, or was the replacement to try and fix the same shaking problem?
It's worth checking the driveshafts again, particularly the inner joints (especially considering these have to come off before you remove the gearbox to change the clutch). Make sure the joint has not worked loose on the shaft - the circlip might have come off?
If those parts were replaced due to damage, it's possible that misalignment of the front end structure has caused one of the driveshaft joints to pop out of the diff, and lose the needle roller bearings in the process. If one of the driveshafts has missing needles, there will be a horrible shaking (I know, because I've experienced it
)
If those parts were replaced to try and fix the shaking, then the only thing left to replace is the clutch.
-Alex