vmrod & LVMYVT76
Nine out 10 front bumpers on '75 and later cars will not fit out of the box, even flex fiberglass. Flex fiberglass bumpers are of little or no advantage in front, since filler is still required on the bumper to fill gaps for a good fit. It's best to use a standard fiberglass bumper because it is a better foundation for filler.
A couple things need to be done to get both of your bumpers to fit over the factory hardware. Once they fit over the honeycomb and rubber cushion, you will have a more accurate idea of the fit to the body.
1. There is a steel reinforcement with a lip on it running horizontally across the top. You need to remove the lip with a cut off disc from the center outward on both sides. This will allow the fiberglass bumper to slide in place easier. Get a helper to hold a piece of steel flat against the body in case you slip with the cut off disc.
2. The big rubber piece in the center needs to be shaved on the top in conjunction with shaving the top of the honeycomb. Both bumper guards on the rubber piece will need shaving as well to make them narrower to fit in the fiberglass guard openings. Once all of these are reduced in size, the fiberglass bumper will fall into place. If you use some childs white sidewalk chalk smeared inside the bumper, you can check for interference. When you remove the bumper the chalk will remain on the parts it came into contact with. You don't want the fiberglass bumper rubbing on any of the inner structure, the abrasion will cause problems later on.
Once all the interference with the inner structure is eliminated, you can start working on bumper fitment. This is time consuming and you'll be test fitting a lot to get it right.
1. The top mounting surface (back side) needs to sanded to match the body contour across the front and between the headlights. The gel coat is pretty thick and a lot of fitment problems on the ends can be minimized or solved in this step. Use a flat block and stop sanding when you reach the fiberglass.
2. Once you have the best fit across the top you can concentrate on the ends. Expect a gap as much as 1/4" on the ends.
Use Evercoat Vette Fill to fill the end gaps essentially making a mold. No filler is needed on the body, in fact there will be no damage to the paint. This method also insures that the body and bumper mounting surfaces match at the same angle. I'm not referring to the visible outside gap, but the gap inside where the bumper and body tighten together. If you were to do a cosmetic fill on the outside, then tighten the bumper to the body and close the inside gap between the 2 surfaces, the top surface of the bumper will distort from the pressure. It may not happen right away, but within a couple months you could have a crease in the top surface.
This process works better the first time with a helper. To fill the gap (assuming the top fits perfectly without force) cover the front surface of the body and the mounting surface where the filler is needed (and slightly beyond) with Saran Wrap. This will keep the filler from adhering to the body and protect the paint. Sand the gel coat off the bumper where the filler is needed and slightly beyond as well as the some of the outer surface. Clean the bumper with wax and grease remover. Vette Fill is kind thin so you need to work fast and carefull. Apply Vette Fill to the bumper mounting surface (one side at a time) and put the bumper in place against the body with very little pressure against the body. Use masking tape to hold the bumper in place and take off the excess filler with a bondo spreader. Let it dry.
Working on the same spot, force filler into the gap from the backside as well. You'll need to verify the gap between the parts is filled sufficiently, add filler to air pockets if needed. Now just finish sand and feather the outer portion. The gap problem is fixed and when you tighten the bumper to the body, there will not be any surface distortion.
