I've worked on S.U.'s for a lot of years, and you're right, most vacuum leaks which cause high idle speeds can be traced to bad gaskets (easy to make your own replacements) or loose (worn) throttle shaft bushings.

There's another problem I keep finding when people bring me their cars; they have attempted to adjust the carbs themselves and in doing so, removed the vacuum bell covers. In replacing them, many times all three retaining screws in each cover have not been tightened equally.

This warps the bell just slightly, enough to cause the piston to sieze in the partially-open position.

Make sure that if you remove those bells you tighten all the screws the same, then lift and drop the pistons by hand to make sure they're not binding up.

Steve