First off, street fiberglass isn't going to be much lighter if at all. I work on those trucks a lot, believe me the fenders and hood don't weigh a whole lot. They are basically only "paint holders". Even the front bumper is pretty light, the rear "Bardon" bumper, yeah it is pretty heavy.
Fiberglass parts are not perfect by any means, they fit like crap and require a LOT of work to make them look very nice.
If you installed all those glass parts, lighter wheels, bumpers, you would save, what? Maybe 300 lbs MAYBE.
All these parts would cost you a ton, is all that money going to pay off? How long?
What would that do, to what end? Save .0008 miles per gallon? Cut your et by .005 in the quarter mile. What are you after?
I suggest if you want to lighten a 99 Chevy full size truck, take the radiator cap off and drive an S-10 under it.