I have a 98 stepside, the box sides arent fiberglass, there made of something called SMC, I had cracks on both sides when I got the truck and wanted to fix them as well. To fix it i used a die grinder with a coarse disk on it to grind out the crack a little and make a wide valley, then I used the same die grinder to sand down the surrounding areas a bit so I could get some "filler" all around and in the cracks. The "filler" I used was a 2 part epoxy in a tube that is meant for these kinds of repairs. I think the stuff I had was made by proform, but Im sure other companys make it too. All I did was squeeze out the epoxy, its a 2 part so I had to mix it on a bondo board, then I spread it in where it was needed, let it cure, blocked it out, primed it with an epoxy sealer, then a couple coats of high build, sanded then painted her black. Still looks great after about 8 months or so.