Original instructions Earl's comments
Since I'm doing a custom build, I decided I wanted a hood scoop on the spare hood that I had. I finally decided on a Shelby GT scoop. With detailed instructions from Autobodystore.com Forums written by Len Stuart, the owner of the site, I proceeded as follows:
Take the scoop and place it in the desired position and draw its outline on the hood.
Cut the metal (for a functional scoop) so that you have a couple of inches under the edge of the scoop.
Drill some 1/8" holes in the scoop through to the hood about every inch and insert a screw to pull the scoop down. (Doing this type of mounting on other areas of the vehicle you may be able to clamp the panels together.)
After the holes are drilled and the screws are in place remove the screws and the scoop.
Grind the metal clean near the point of contact as well as the glass using a 16 or 24 grit disk.
Apply your bonding material (I've been using Dyna-Weld) To both surfaces pushing the material into the scratches then build up a little on the surface and put the two pieces together and screw it down. DON'T screw it down tight because the Dyna-Weld is your glue and if you squeeze it all out your scoop will pop right off. Just pull the scoop down enough so that the paste starts to push out of the crack a little.
(Earl: Since the Dyna-Weld was a one-to-one mix ratio product, I used 2 matching caulking guns to dispense. A number strip comes with the Dyna-Weld kit to apply to the plunger handle of the caulking gun for more accurate dispensing.)
After the material gets hard remove the screws and grind off the excess bonding material and do some initial shaping with the grinder.
Countersink the screw holes and cut a strip of fiberglass cloth about an inch wide to cover the seam all the way around the outside of the scoop.
Using more Dyna-Weld apply it to the edge filling the screw holes and giving you some buildup over the seam then push the fiberglass cloth into the soft paste so that it covers the seam and apply more DW on top while everything is still soft. You may need to do about a foot at a time because this could take long enough for the DW to start to harden. So cut the cloth strip into 1 foot lengths and do one at a time until you're all the way around and the cloth is buried in the DW and covering the seam.
Once the DW is hardened over the cloth use the appropriate sanding block to shape the filler. Apply more of the filler (you can now use flexible polyester putty) and continue the shaping but try not to sand so much off that you sand through the cloth reinforcing strip.
(Earl: The picture below shows some of the tools I used for the shaping and sanding of the material to create a smooth transition betweeen fiberglass and metal.)
The idea here is to use a flexible material (Dyna-Weld and flexible putty like Poly-Flex) so that the different expanding/contracting characteristics of the materials won't cause cracking.
When you have your shape sanded to where you're happy with 80 grit paper then go over it with some 180 and then spray it with some 2K primer using a flex additive so that the primer is also flexible. After guide coating and sanding the primer with some 400/600 grits sandpaper you can apply paint but be sure that the paint ALSO has flex additive added so that it won't crack.
(Earl: Pictured below is the finished hood covered with Kirker Euro epoxy primer. I really like this primer and it is easy to sand when necessary.)
(Earl: I deviated from the instructions above using materials I had on hand. The fillers I used were Evercoat's Rage Gold and UPC Icing. The picture below is the back side of the hood showing how I did not sacrifice the integrity of the original hood bracing. I'm hoping this will prevent the flexing issues that others have had. The color is the color that the Ranchero will be when finished. I hope my deviations from the original instructions won't come back to haunt me later.)
Thanks, Len, for the informative write-up. If anyone would like these instructions, they can be download here.