I opted for a fiberglass 4" cowl hood, and, with a 32 pitch upper pulley, the belt and upper pulley are inside the hood (big block Chevy). I cut the hole smaller than most I've seen, but I do have to remove the scoop to open the hood (no big deal). I just thought it would look a little cleaner.
Here is how I cut my hood:
With the hood off of the car, I simply used string and painter's tape to mark the location of the blower in relation to the fenders, cowl, and front valance.
First, tape the string on the cowl and then to the front valance along one side of the blower, taking into consideration any linkage, fuel lines, etc. Repeat on the other side. Second, tape the string on the fenders, one to run behind the blower, and one in front, across the engine compartment. This will give you the outline and location of the hole. Measure the distance from the corners where the valance and fender meet to each of the contact points of the string. Take into consideration that there will be a gap between the hood and the body panels. Do the same with the cowl/fender corners. Measure the same distances on your hood, and tape the string across the hood from those reference points. You should end up with a rectangle in the center of the hood. Measure AT LEAST twice to ensure the reference points are equidistant from the corners of the hood. If you need to cut for clearance for the front pulley/belt, you will have a reference point by the position of the rectangle. Use a straightedge and a sharpie to outline the hole. I used a small ( one inch diameter i think) hole saw on a drill to cut the corners, then a dremel cutting wheel to cut from corner to corner.
There may be an easier way, but this worked for me. Hope it helps, and good luck!