I would recommend using standard bonnet and go for lightweight bumpers tailgate etc.

The standard ford metal bonnet at highspeed flexes so i didnt even bother using my fibreglass one i had.

As for bumpers i would use standard ones as well and strip the reinforcing metal frameworf that is hidden under the foam filling on the reverse side of the bumpers. You will be surprised how much weight this saves.

I think the amount of fibreglass you will need to make the bumper sit and bolt nicely to the shell will out weigh a plastic bumper which is stripped out.

The tailagate is also an area in which i have saved alot of weight through using a fibregalss tailagte.

The biggest saving of weight has to be windows.