They installed A/C systems with the big block from the factory, so you should have few fit issues. The real issue is where you will put the evaporator unit in under/in the dash. The basic question is did you want to go stock, with the stock style compressor and in dash A/C? or do you want the more efficient aftermarket compressor and under dash units like those from classic air? Actually, there are lots of variations to compressor type and placement, brackets, refrigerant (I suggest R134, since you are getting a system, not retrofitting an existing system). I have a friend who is the original owner of a 1967 T-500, and he had a small, efficient Sanden compressor and in dash unit installed in his car. The compressor is on the passenger side, IIRC. If it fits with a 428 with two 4V carbs, I'm sure it will fit with a 390.
Daniel