Use the mounts that match the cab.
Are they both auto or standard trans misions? or one of each? Your might need to put the 4x4 floor center section in the 2 wheel drive cab. If the 2 wheel is standard and the 4x4 auto you will need to swap the steering column or find one from a 79 auto as the 84 is most likly for a 4 speed auto. No biggy but the indicator will not read correct. If they are both standard you could still have a bit of work as I have had several 84s some hydraulic clutch and some linkage. Most of the 84s I had were linkage but had two that were built after the 10th month and they had hydraulic clutch. No hole in the floor and the clutch pedal itself is diferent. A linkage pedal will swap in no problem.