Fiberglass to metal is a sticky area because of adhesion. I would try to locate a metal cowl scoop, there is a company making these I just am unable to find thier ad. They advertise in car craft mag. and also have a web site. If you already have a scoop and are determined to use it here is what I would do to achieve the best results.
Trim scoop to fit as close as possible to hood all the way aroud .
Grind the surface of the scoop bottom and the area of the hood where it will rest.
Drill several 1/8" holes thru scoop flange and hood
Use a good two part epoxy structural adhesive( 3m, Lords, SEM)made for bonding fiberglass to steel.
Apply glue to flange bottom and position on hood and rivet with 1/8" rivets .
Allow this to dry 24hrs. make sure it stays 65degees for 24hrs this is important.
At this point you can flip your hood over and prepare to cut the opening in the hood with a jig saw.
Don't remove the back edge of the hood without installing some type of brace, this scoop is the weakest piont on your hood.
Remove the rivets and fill with your choice of filler.