I am planning to have the UltraRacing rear anti-roll bar (20 mm) and frame brace done to my 2002 XKR coupe but wonder if the $450 repair for my gas tank float should be done at the same time or before?

Anything else that should be done at the time those parts in the rear get done?

 

The fuel tank is in the boot, so there isn't any real relationship to the rear suspension. Up to you whether you want it fixed now or later.

You might also replace the panhard bushings with powerflex ones, but if you expect to some day install a LSD, that would really be the time. The easiest way for the LSD is to drop the cradle which makes all the bushings and shock mounts and everything else very easy to get at.