I have a 79 that I got a flex fiberglass front bumper cover to replace the bad urethane cover that was on it. The new cover is a bit short on the sides and am also considering filling the gap now that I have the fiberglass cover. What filler and prep do I need to use to make sure the gap fills properly and doesn't crack. I've done metal body in the past work but never the fiberglass.

 

If you are trying to build the sides out a bit Evercoat SMC Resin and mat should do it. If you are trying to fill a gap Vette Panel Adhesive/filler should work. Both set up pretty fast.

You have to make sure you use the right resins on the SMC fiberglass panels.

If you don't get more suggestions here try posting in the body/painting section. A gentleman named Dub is generally there and has a world of experience when it comes to body work/paint on Corvettes.