Air was not as common back then, especially in your region. Disc brakes were manditory with a BBC.
That is true for '69's, but disc brakes were an option in '68 on all models, and relatively rare with less than 13,000 of them sold. '68 used the 4 piston calipers, '69 and up had single piston calipers.
Only the total numbers for options are available, it isn't broken down by the model (Malibu, SS, Elky, etc.).
Fremont-built cars were the only '68's that have been found with a build sheets, the other plants used smaller Body or Chassis Broadcast sheets. These papers have been found in kick panels, behind door panels, stuck under the dash, and probably many other places.
Approximately 3/4 of all Chevelles built in '68 were non-factory A/C cars.