The Parts Place valance is made of thin, black ABS plastic of even thickness: 1/16". The upper mounting flanges are way too wide to fit in the locating notches in the underside of the bumper.
The factory polyethylene valances were molded in body colors and vary in thickness, from way too thin: 1/32" (on the sides where they mate with the lower front fenders, to thick on the bottom 1/8" where they attempted to mate with the rad support (but the holes were in the wrong places). Polyethylene is a waxy, easily scratched polymer material. They also have several molding seams on the inside.
Here's a photo of the original white poly valance I repaired for my white 72 T/A next to the black Parts Place valance. It was completely shattered and burned but I got it repaired better then the blue one I just used, so I put it back on the white T/A.
Here's a portion of my old thread on repairing that one. It has a lot of detail shots of the poly valance:
http://forums.maxperformanceinc.com/...ne#post3331725