I will tell you exactly what NOT to do.  Don't bother with the factory AC stuff.  There are no more good cases left for these cars.  What's out there is hacked up, half @$$ed fiberglassed back together by some hack, and sprayed over.  The casing halves don't seal right, the brackets are misaligned; long story short its all junk. 

The old style compressors can be made to run on R134a, but they aren't designed for it, so efficiency suffers.  They also eat horsepower to the tune of 20-25HP to run them.  So, IMO the old style compressors, if you don't already have one, are not worth it.  You want a system that uses a Sanden or one of the Denso compressors, which use like 3-7HP to operate and run R134a without issue.

The old style controls were vacuum operated, and the system is problematic due to age and lack of available replacement parts.

Now that I've told you what not to do, let me tell you what to do: buy a modern direct fit system from Classic Auto Air, etc.  '68 had in dash AC from the factory as an option, so there's room in the dash to make it work easily.  The older cars its a real challenge.

The only thing to bear in mind is if you want to run V-belts or a serpentine system.  If you have or plan to have a 5.0 swap (the Fox generation engine, not the new Coyote) at some point you'll be bucks ahead to just swap over to that front accessory drive system now.  In which case you could run the 6P148A Denso compressor that those cars had.  There are also brackets to allow retrofitting a Sanden compressor into the Fox bracketry, but they're getting harder to find.  I personally like the Denso.  You can usually find one for less than $50, and with $35 worth of parts they can be rebuilt at home with basic tools. 

Dave,
 
Look at VIntage Air and Classic Auto Air for your 68.  Compare the two products and see what works best for you. Several folks on this forum are using both brands and will likely chime in soon with good feedback.

 

My wife loves our Classic Auto Air "Perfect fit" system - -  twas her idea. It blows like cold arctic wind.
 I really liked the way it installed, well engineered except for the very rough cast iron 3 sheeve crank pulley and ugly compressor mount.

6sally6 wrote:

Rudi wrote:

except for the very rough cast iron 3 sheeve crank pulley and ugly compressor mount.

Sounds like old Rudi may be "break'in out" the old lathe and milling machine for some "custom" mounts and pulleys
this winter!! (AFTER me comes to the beach for a few weeks)
6s6
 

Yeah Mike, those "custom" parts were done during the A/C install.
 We will be at the "beach" October 1 for a month, looking forward to riding shotgun in the "hot rod"