Trying to adapt another system to fit your fridge would not be an easy task, I think.

Your donor vehicles' AC would have been incorporated into the heating system, I believe. The Fridge, would not have been. It was an under dash system, separate from the trucks' heating system.

Much easier, though still not inexpensive, to use an aftermarket system.

I do have an NOS, under dash, system I hope to one day install. Sorry to say, it is not a Ford unit. I will still need to buy all the under hood components. Since it is so old, I will have it gone through, all seals renewed , etc. It will remain an R12 unit, as the cooling is so much better. More expense, because the R12 refrigerant, while still available, is sometimes tough to source and is costly.