I made this swap 10 years ago, or more, into my '86 F250.
You would think that Ford would use the same heater box, and that the holes thru the firwall would be the same, but no.
The heater box, and the plenum on the insude of the truck bolt together with the firewall sandwiched in between them. Make very careful measurements on the donor truck before cutting/ enlarging the hole in the firewall. IIRC, Ford built enough tolerance into the system so that you probably need to be accurate to only 1/2" or so. The problem comes up when you go to mate the plenum to the rest of the ductwork.
I'm not trying to discourage you--If I did it, surely anyone with the desire to do it can. Just be as precise with the measurements as possible. You will see what needs to be done as you disassemble the donor truck.
I did the heater/plenum, and vacuum controls in one stage, over a weekend. Then the compressor/ condenser and hoses. The final weekend was everything else, evacuating, and charging the system.
Good Luck.
( I may have posted more information right after I did it over on the old ford-diesel.com site--who knows, it might still be floating around somewhere. It might be useful because I probably posted more details, as the job was still fresh in my mind)
