Why would the DS need to be changed, the rear end mounts in the same spot, the housing is the same for disc or drum. As long as he has the reluctor ring for the 3.08's it should be fine
You will need to flash the PCM to correct the speedo/odo.
Your master cyl is the same, that's why everyone does the stealth bolt mod, because GM used the same MC for disc/drum & disc/disc.
Lets not make this anthill into a mountain. Providing all mounting bolts come apart without issues this should be an afternoon job in the driveway as far as old one out /new one in, even if you are not changing the whole housing it's self, any side work...I.E. brakes and stuff will of course take a little longer.
That's my story and I'm sticking to it.