You would have to reduce the interior of both to a steel hull and then put the wire loom down on the floor, over the A/C box, and then bolt the '95 dash back in the '99 cab, then plug in the wire loom to that dash, then install your '95 steering column with key switch, and plug in your seats, air bag, overhead lights, etc. , out of the '95. All the '99 wire loom would have to go into the '95 cab if you were to want to sell it. Then you would have to switch the cabs and plug in at the firewall and reinstall your brake cables. I believe the '95 and '99 cab mounts were the same, so there should be no problems. You can even remove the whole cab with the front fenders installed if you have the equipment, and set the whole thing on the '95 frame. The wire looms will not interchange. The '99 has quite a few more circuits and the wire looms will not plug into each other. Also, the external wire loom on the front of the cab in the engine compartment on the firewall will have to stay with the engine and frame of the '95, along with the PCM, to await it's counterpart internal wireloom to be installed in the '99 cab and be set on the frame. That wireloom won't even fit a late '96 or '97. The firewall will be close to matching where the wireloom passes through, but that may be a slight hangup, with need for minor modification, depending on what wireloom your '95 has. The hole the block clips in may be bigger on the '99.
And if you have power mirrors, windows, and doorlocks, you'll have to remove that wire loom, as well, and send it with the primary loom. They won't switch out. You could splice and fit, but I think the results would be somewhat disastrous in the end... ...

All in all, it's not an easy job, either way you look at it. I think for reliability's sake, and your own sanity, pulling the entire wire loom would be the best choice. While time intensive, it would make for the best end results.