My experience is that ANY fiberglass part from ANY manufacturer needs work. By work, I mean cutting, repostioning, re-fiberglassing, then filling and sanding. Some need lots, some only a little, but it all needs work. My best luck has come from parts I bought from Mustangs Unlimited. The worst has come from Maier and Branda, in that order.
+1 on the mustangs unlimited apron... the one ive got only needed a good coat (o.k. three coats) of 2k primer, and block sanding. you will most likely have to notch the lips of the fenders to make them follow the contour of the apron. after you are happy with the fit you can either weld, or epoxy a backer in with the right contour to give the finder lip its strength back.
I bought one from MP years ago an it sucked to put it on. To make it worse now scott drake sell them for the 65-66 and fit is unreal. So I'm pissed now because I put all this work into something that fits like ass. Dyna Cor makes them for SD. I sell them all the time and they line up EXTREMELY well, especially for glass. It isn't a plug for me, so don't buy it from me. but find a vendor that does, and a Scott Drake.