Ac in a 1975 dart

So you have no parts from the fire wall to the rad support ?
If so a firewall forward kit from Classic Auto Air would fit the bill.
This will get you modern Sanden compressor and proper attaching hoses.
That would require that the under dash parts are still there.
Not sure if they supply the pulleys and engine brackets complete or not. See what they got.
72 LA engines have the same aluminum water pump as 75.So all should bolt right up.