Simply for the reason that it's much easier to do.
Looking at these pictures, I changed my mind. Go lead, screw the fibreglass.
Your car looks great!We use Duraglass for a base and then filler. All 3 of my current cars were done this way. I race all my cars. 12.5@110 stick 10years,11.0@123 auto 25 years, 9.25@147 Auto 2 years. None have cracked. All have very nice paint.
Doug
I would choose lead if I had the stuff and knew how to do it but I don't so I welded my roof seam solid, filled it with glass and finished it with regular filler. Maybe my next car I'll learn how to do it.