Without seeing the truck, I can only suggest squareing up the cab on the frame first. Do this by taking a X measurement from the back side of the cab to the frame rails, at some point on the rail, such as a rivet that is the same place on both frame rails. Also measure an X on the front side of the cab too. When you get the cab square, you may find those fiberglass fenders were molded from some alien spaceship fender. I hope not, but I have seen some molded from bent steel fenders that weren't close to fitting. If the fenders are off bad, look for some more, instead of making the cab fit them.
