Air Ride has quite an array of suspension bits for the early 60s full size GM cars. I've done a little bit of research, hoping someday to restore my Mom's '63 Buick Wildcat Convertible (same chassis as the Impala).

I recall a TV episode from 'My Classic Car' or other similar show, which covered a large track day/dragstrip meet loaded with muscle cars that had air bag suspensions. From a spectator's point of view, the cars seemed to handle quite well for their heft, pretty minimal body roll. Like Chris said, I would look for some classic car forums for opinions from people with first hand experience.

I don't know what to say about coil-overs. A 60s land yacht is not going to be a track star, so why not leave it a nice cruiser with some moderately updated handling. To me, air bags sound like a good means toward that goal; if that's what your friend wants.