Last year, I installed a well-made steel repro dash rail (Snyder's, I think). It cost somewhere around $150.00 as I dimly recall. It came with about 1 inch of extra length on both ends so it could be properly fitted against the windshield stancheons, the contour of the top of the gas tank and the forward edges of the doors (no two Model A's are exactly alike as we all know).
That was one nasty job! - cutting, grinding, bending, forming - many times (remove too much naterial at a time and you're in deep trouble!) to get it "perfect" - and getting the mounting holes all to line up with the under-structure properly. But I felt good for the experience and the feeling of knowing I fought the good fight to stay with an all-steel body. 'Just a personal committment I made in restoring my car.
After getting the rail to fit right, it then went to a good body/paint shop to be filled, primed and painted. It now looks great on the car but I wouldn't go through that again. I trial-fitted an original rail that I borrowed (before committing to buying) and the fit was just not right with the stancheons and door gaps. It would have ended up needing "altering and customizing."
So if you have to do "custom" altering and fitting anyway, whether you buy original, fiberglass or steel repro, why not go with steel which is the correct material in the first place an is easier (relative term) to work with and shape?
The question is - Do the fiberglass rails come already "shaped-to-fit" - "one size fits all"? I can't immagine that being possible with the dimensional variations that exist among Model A's today. And fiberglass can't be bent and "formed" to a slightly different shape - and hold the new shape - to help achieve best conformity with the mating surfaces.
Go with a good steel repro and enjoy the practical sheet-metal-working experience you're going to get.
My 2 cents....
Earle
