Originally, for my hood, I'd bonded an old, cheaply made fiberglass hood skin to a metal frame and it worked for a time.  However, after a while, the ungelcoated thin fiberglass started to print through.  Plus, the flimsy fiberglass just made it untenable.  So I got an original steel hood for $100 and one of those new steel Shelby scoops and welded the two together.  The original steel hood was pretty clapped out and required a lot of body work to be usable, so I didn't feel bad about cutting a hole in it.  Below are a few pics about how I did it, using a Lincoln 140 MIG welder.  I butt welded the metal, using 0.024" wire and the usual 75/25 gas mix.  As everyone knows, the biggest problem is preventing warpage.  I took the usual approach of tack welding, skipping around the perimeter, but used butt clamps and kept wet paper towels around the weld.  The result was very little warpage.  The other thing I did was to extend the hood's sheet metal under the scoop until it met the bracing, unlike the typical approach where the hood's sheet metal is cut off right at the opening.