Minor correction:
There are 2 rear sections:
short (found in Regular Cab Short box and SuperCabs' for the 6' box...i.e. about 99% of all Rangers)
long (found in Regular Cab Long box; 6" longer behind rear wheels; for the 7' box, rare..)
There are 6 front sections:
Regular Cab Short box, coil springs (exclusive to 4x2; for 6' box)
Regular Cab Long box coil springs (exclusive to 4x2; 6" longer behind cab; for 7' box)
SuperCab coil springs (exclusive to 4x2; for 6' box)
Regular Cab Short box, torsion bars (4x4 and some 4x2s*; for 6' box)
Regular Cab Long box torsion bars (4x4 and some 4x2s*; 6" longer behind cab; for 7' box)
SuperCab coil torsion bars (4x4 and some 4x2s*; for 6' box)
*Technically the torsion bar 4x2 frame is missing the mount for the front differential, and it does need to be cut out of a 4x4 frame and welded in, but my assumption is if you are up to a frame swap, welding one mount would not be beyond your (or a friend's) ability.
It's also not a safety thing - if the mount broke the front differential would just be able to rotate a bit and make bad noises, not anything catastrophic.
If you want to build a SuperCab long box, you need to either add 6" to the long box frame or 6" to the SuperCab frame (or both to double the overlap).