Air shocks are likely to give you a rather unpleasant ride. They also _can_ exert excessive load on the crossmember the tops bolt on to; it was not designed to handle those stresses. I've seen them rip out (on my car, which had nasty hard shocks on it when I bought it; damage already done when I put normal hydraulics in their place).
Past threads on this have tended to recommend going with replacement springs (ESPO?), perhaps ordered with a higher arch, or getting your springs re-arched. Longer shackles seem to be considered bad, and can cause damage to your rear valance in some cases.
