The original was not fiberglass. I don't think the original was ABS either would break too easy. They were pretty flexible to prevent breaking on parking curbs. If I remember correct they were about $18.00 new from the parts dept. I have one NOS left in the hoard for one of my cars.
I would think fiberglass would vary in size more than a molded part.
1971 Mach 1, Grabber Blue w/Argent stripes. Original 2V 351C Auto, Tilt, rear defog, Black Comfortweave Interior. Under restoration. Original colors, 4V 351C, 4-Speed, Spoilers, Magnums, Ram Air. All Ford parts.
only real issue I've seen with the ABS is it is harder to get the paint to stick to it even with flex additive to make it argent.
one buddy of mine had it painted and the paint kept peeling off so he bombed it with krylon plastic fusion paint that was close to the right silver color and the paint finally stuck to it. the color does not 100% match his rockers but it looks good.