The stock GT5 came with fiberglass. I have had them for 27 trouble free years. Not a crack or a chip or any other problem.
I've have seen the steel fenders and they are totally inconsistent - some good, some bad some horrible. And all will require a lot more work to fit and many more mods to your awesome paint.
My two cents.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.