I've never actually added a fiberglass scoop to any of my cars yet, but I have seen them done a few different ways. I have cut an opening for a supercharger before, but we never added a scoop. As OUTLAW mentioned, take your time figuring out location. A mistake there will ensure you won't be happy when you're done.
You may want to take some measurements first of the location of your carburetor(s) in relationship to the cowl and fenders because it's difficult to tell the carb's location in relationship to the hood when it's closed. Use tape to make marks that you can reference. Not everyone locates their scoops in the same spot. You may want to choose the location partially based on cosmetics. - What looks good to you. Or you may want to locate the scoop in an area that corresponds to a tray that will seal against the bottom of the hood. I've seen plenty of guys mount non-functional scoops that had no opening into the engine compartment.
Once you've figured out where you want the scoop to be you have options on how to mount it. If you're not planning on cutting a hole in the hood you could just pop rivet it in place and leave the rivets showing. Make sure you space the rivets evenly if you do it this way because it will affect the overall cosmetics. There are some really strong adhesives on the market that might eliminate the need for fasteners. If you want the scoop molded onto the hood you may be better off not having rivet heads to contend with.
Hoods get a lot of heat and fiberglass does not expand and contract at the same rate as steel. Eventually a molded on 'glass scoop is likely to crack partially due to vibration and partially due to the differences between the two materials. That is why some guys choose not to mold them in.
If you are going to cut a hole in the hood, study the understructure first. You may want to cut a smaller opening instead of cutting an opening as large as the scoop. The smaller hole may be easier to seal around if you're going to go with a bottom tray or if you don't want a bottom tray that is so large that it covers most of your motor.
If you do want a hole in the hood but are not planning on an underneath carb tray that will seal to the hood you still have options on how to make your cut. You may want a full sized opening that corresponds to the size of the base of the scoop. You can use several methods of marking the hood. I would lay some masking tape down around the perimeter of the scoop's flange. Then I'd pull the scoop back off and make marks on the hood 1&1/2" inward of the inside tape edge. (you said the scoop had a 1&1/2" flat lip around it)
Some guys will just pop rivet it down on top of their hoods. Some will use small stainless steel allen head bolts. If you choose to use bolts you may want to use nutserts so that you don't have to use nuts and washers on the bottom side.
One other thing to consider is the contour of your hood. I don't know of any hood that is completely flat. Before you commit to any cutting or mounting, look to see how well the scoop sits down against the hood. Fasteners will allow you to bow the scoop somewhat, but if your hood has a large crown to it you may have to modify the scoop to get it to fit.
I have seen flanged scoops such as yours installed from the bottom side too. I think some guys do that to avoid the amount of bondo and fiberglass necessary to mold a scoop on that they'd need if there was a lip on top. If you choose that method you may have to cut the inner structure a bit more to allow room for the scoop's lip.
