Roof rust is an issue in all of them as the factory didn't coat the metal on the inner side of either the outer or inner roof skin on any of them
Look for evidence of Ziebart or Rusty Jones treated cabs The dealer some times sprayed the inside of the roofs on some to kill water leaks
If you find a good one spray the insides of the roof before you put it on the truck so you can turn it & use gravity to your advantage
You're limited to 81-85 cab due to not only the HVAC but the steering column mounting changed in 79 also
