If you are going to use a body filler, I recommend against Bondo or fibreglass. Repair metal with metal.
I would use either Allmetal (a two-part metal-based body filler, which is readily sandable,) Labmetal (rather tougher,) or VersaChem's two-part steel stick. No matter what, you really should repair metal with metal, and save fibreglas for the liftgate or your boat.
Allmetal is found in most well-stocked paint/body supply houses, and Labmetal should be at your local welding supplier. MSC Direct has Labmetal as well, as I recall.
Note that Labmetal is REALLY tough - I've used it to repair mistakes made by "shade tree" machinists on parts with light to medium stresses without difficulty. Miles better than JBWeld, too!
5-90