Two questions

I have a '93 4x4 step-side and the fenders are metal, but who knows what they've done in later models. I wouldn't think it would be a great idea to use fiberglass in situations where huge rocks will be flung up from off-road use!

I also have a 5.7 liter, but a guy I work with has the 5.3 in his Tahoe and it blow my doors off! As for holding up, it's probably too early to tell, but the power is definitely there!

-Derek

My 03 Sierra Denali has plastic side panels/fenders on the rear. It's not a step-side in the usual sense; it has a full size bed but the fenders have to extend farther out than a normal Sierra to accomodate the 4 wheel steering, so it has wheel-well bulges like a stepside. No issues at all, except I find that I get bug splatters on the rear fender, something that never happened on my normal fleetside GMCs. I haven't had it long enough to be sure that rocks won't kick up and damage that fender as more time/miles go by.

HMc