My 1970 Duster 340 FM3 project

Are you going to try for a Factory AC unit? I would if I were you.

No, I am going to go with Vintage Air or something similar. Modern compressors use less power and the evaporators actually work better as well. Plus, the lines feed usually through a bulkhead placed where the factory blower motor goes so no firewall modifications are needed. Also, if I decide, I can also use a low mount compressor that is virtually hidden from view so that the engine will look closer to stock.