A couple of people asked so I thought I would post.
Disclaimer, I'm not a mechanic nor did I know I was going to post, so my appologies on teminology and how detailed it is.
If you are looking for a box, I suggest searching car-parts.com as there are hundreds of boxes out there thanks to the rotting frames. All boxes for 1rst gens, fit 1rst gens, 1995.5 to 2004.
Mine was mint and I got it for 600 bucks!
1. Take of tail lights, it's just a few screws and some clips.
3. Take of "ears" of the stepside bumper, no worries a regular bumper awaits under there! I've got it taped here to not scratch the new box. (pic is with new box but you get the idea)
4. Remove other extras like fender flares (they will fit the fleetside) and tailgate as it will be easier to deal with
5. Remove gas tank collar, it is different and you will need one with your new fleetside box.
6. Undo the six bolts that hold your box to your frame and take the box off. Note, stepside box is a lot heaview than the reg box.
7. place the new box on the frame, alligne it up and bolt it down.
8. Put tailgate, lights and gas tank collar back on.
9. Put your extras back on, like the fender flairs.
While the box was off I took the opportunity to spray the inside of the wheel wells with bed liner, and I sprayed Krown, rust proof inhibitor, everywhere I could.
I also had to swap my mud flaps over and bought new trd decals as the doner box came from a 2wd.
I aslo did a roll in bed liner, herculiner and seems to be holding up well.
Hope this helps VanTaco?