I am adding A/C to my '74 now.
The options out there are:
1) Find all the pieces to a dealer installed system and install it in to you teener ($400-$600)
2) Buy an updated universal aftermarket A/C system from someone like Vintage air. ($800-$1000)
I am installing a dealer installed system into my 914. I am upgrading to a Sanden compressor (you will need a bracket adapter), new hoses, new fan, new drier, new binary switch, and will be using R134a.
The evap and condenser will be the original ones used for the dealer installed system. The condenser will be upgraded at some point in the future
Cheers
Kerry