This isn't about a Burb, but a friend of mine who's a bodyman used a fiberglass fender on his personal '57 Chevy. It looked pretty real, and not being a bodyman I couldn't tell the difference without a magnet. The Burbs you're talking about are pretty rare and a pleasure to see on the road.
If you can't find an original or NOS part, I would say go for it.
Just my two cents.
