I sort of did this, just heat though on an 80.

The box fit the firewall and kick panel without much effort.

But the 81 up ducts are molded to fit the 81 up dash, vs hose for the 70's stuff. So...

I swapped the newer dash, which I don't recommend. The only holes that lined up were under the windshield. The lower corner bolts are nowhere near the right spot and the steering column mount is slightly higher on the older truck.

Then there's wiring, the newer ammeter is not compatible with the older wiring.i ended up swapping the wiring harness as well which has a different bulkhead connection.

Remember, this was heat only, ac probably adds more headache.