Adding factory AC to a 77 grand prix
 

I picked up a very nice 77 grand prix SJ for a very fair price. About the only option it doesn't have is AC, and i'm considering attempting to add it. What am I getting myself into?

If it’s a SJ it should have a 400 Pontiac engine. I’d say the easiest way to install A/C would be to find a donor 77 G/P that has A/C and strip all the needed parts. Take lots of pictures, it will help.

Firewall opening - heater vs. AC is different, to best of knowledge on Colonnade cars. You might wish to begin researching aftermarket systems. Less weight, clutter and may in many cases be more efficient.

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.

It is a 400 pontiac, and unfortunately did not have factory ac. From what I've been able to determine, going aftermarket is the way to go as you gentlemen suggest. I'll see what's out there. Thanks!