I fitted air conditioning to my 740td from parts of several donor cars.
The most difficult part is the wiring - the electronic climate control system (ECC) has sensors in several places. We separated the wires from the donor car loom and made them into a separate loom in my car.
As you have a 960 you may find the wiring is already in place, which will make everything easier. I'd imagine you'll need to remove your dashboard to get the heater box out - that'll take a day to remove and refit.
To my mind it was well worth it to get the spec of car I wanted
I found this very useful for understanding how things work.
https://www.volvoforums.org.uk/showthread.php?t=97912
While you have the system pipework apart, replace all the o rings