Air shocks were not offered by the factory... not even in a tow package. Stiffer springs (and up thru 77 a larger rear sway bar) were what the Y96 towing package (called Firm Ride Pkg) used to add extra stiffness to these cars... and to what extent the spring rates were increased depended on whether the car was an auto or 4spd.
The shock mounts are not designed/meant to provide the extra stiffness. Air shocks have a habit of breaking the shock mounts if really abused... not a good scenario. :lol:
Air shocks were most likely put on for 1 of 3 reasons... 1) for that classic jacked up hot rod look, or 2) because the rear springs had sagged so bad, and putting air shocks on was cheaper than replacing the springs, or 3) because the factory shocks had gone bad and air shocks were seen as an "upgrade" back in "the day". :lol:
My car has air shocks right now... they are definitely not original. They are/will be coming off pronto in the resto and replaced with a full set of Konis.