To correct a couple minor errors:
1. '93-'11 Styleside 6' boxes are all identical from an aesthetics point of view.
1a. '08-'11 Styleside 6' boxes have additional bolt holes (8 to 10 vs 6 in the older Rangers, you may decide if you want to add holes to your older Ranger frame to add bolts, just have drain holes or weld the holes shut).
There are differences in the wiring harness between '93-'00 and '01-'11 (amber versus red turn lights)
The above ignores the minor differences in fender flares/ bed caps
You may drill the holes if your box doesn't haves them and you want flares and/or bed caps
You may weld holes closed and repaint, if the box has them and you don't want flare and/or bed caps
2. '93-'04 Splash boxes are all identical from an aesthetics point of view. All Splash boxes are 6'
Again there are difference in wiring harnesses been '93-'00 and '01-'04.
3. '02-04 Ranger STX and Mazda B2300/B2500/B3000/B4000: These boxes are also '6', but have slightly different style. They use the '93-'00 wiring harness (amber turn lights)
You would need to modify wiring harness to fit on later truck (and maybe paint the amber plastic red).
The 6' box may be found on regular cab Rangers and Mazda Bxx00 with short box and SuperCabs (Cab Plus in Mazda vernacular); the 7' box is only made in Styleside; and Ford only installed on regular cab.
Note 1: The bumper from Splash is narrower than than that of Styleside so if you care about aesthetics you want the correct one and there is a minor difference in length of fuel filler pipe. And Mazda B2200 is different all together
Note 2. The fenders from Splash unbolt from the 'box'; so if you fenders are good, all you need is a good 'box. Conversely, if you damage a fender but the 'box' is still good, all you need to do is unbolt the fender and replace with a good one.
Technically the '83-'92 Styleside boxes will bolt onto later Rangers, but there are major aesthetics differences.