On a '92, the OP will need to shorten the frame in 2 places: 6" before the rear wheels and 6" after (as part of this, you may need to relocate the spare tire crossmember). You have to shorten the rear half too as the bottom of the tailgate is lower than the top of the frame; so you wouldn't be able to open the tailgate if you don't.* And you of course need a short bed driveshaft, shorten piping to the emissions canister and gas.
*Notching the back 6" of frame (rather than removing section) would leave it very weak - not good for towing, or in case of accident.
On '98+ Rangers, you only need shorten the frame in front of the rear wheels as the frame separates just behind the cab; and then you can use a supercab/short bed rear frame section.
My '98 Ranger is the opposite; I added 6" of frame just behind the cab; and a long bed frame section to make a long bed supercab. I had to add an extra bit of piping to the gas tank filler**; extend the rear brake line (I have a Superlift as well so added 12" - 6" for the frame extension, 4" for the lift + some slack.) and there is a 6" extension to the charcoal canister, a 6¼" longer driveshaft (square/hypotenuse thing, didn't have to add 10") and a 12" wiring extension to the rear lights.*
*While I was modifying the wiring, I changed the trailer plug from 4 wire to 5 flat, and used the blue as back up lights (Allows activation of the solenoid on my trailer's surge brakes, so I can reverse without trailer brakes activating). A 4 flat plugs into the 5 if I am towing a different trailer without the extra pin.
**If someone can tell me how to "repair" a plastic gas tank, I have 2 tanks which I could graft together into one longer one. The 3 extra gallons which a 6" longer tank would net me isn't much, but it is 20% more...
If you were '93+, I have a slightly experienced long box for sale on driveway.