Corvettes have pretty straight and flat hoods, so it must be doable. I would think that bracing and support would be a good part of making a fiberglass hood nice and flat. On my own 442 project, I used an aftermarket hood and, after a good deal of work, I believe it is pretty flat.
As for fiberglassing in general, have a look at West Systems epoxy based products. Very nice stuff, and in my opinion much easier to work with as compared to the polyester based products.
http://westsystem.com/
Shiny Side Up!
Bill