[Trevor78]

No, the 400 wasn't the only engine in a '77 SJ. Best to confirm what it is first. Also, check that the AC hasn't been removed, rather than not optioned as these cars were fairly high up in options. For an original non-AC car I'd probably look at the modern AC kits to suit, they already aim them at heater-only cars. I can't suggest anything directly for the Grand Prix but the big names may have a suitable kit. Unless it's a Chev or a Mustang, most kits require mods even if they say they fit. If it's an original factory AC car with it simply removed, I'd be going back that direction, with upgrades.
Weight reduction may not be instantly on the mind of a GP owner. The testament to the solid quality of the A6 compressor means that 40-50 years on, they are still working, something the more 'efficient' light compressors rarely have. Each has their place based on what kit you install.