The air dam serves multiple purposes. Primarily, it will create a little down force to plant the front of the car better. The flip side of that, of course, is that it also creates drag. Tough to get one without the other.
As you said, it also keeps some of the air from getting underneath the car and creating lift. Unless you have a belly pan and diffuser, it's less helpful than you might think.
You can also create a dead zone directly behind the bumper; where your
oil cooler lives. It's like blocking off the back of the cooler, making it significantly less efficient. You'll need to duct the back half of the cooler to make it work again.
With a sort of dead zone underneath the car, you need a place for the air to go after it passes through the radiator. The only place it has to go is down and back - which again creates lift. The best aerodynamic answer is the what they did for the GT40 and Daytona Coupe. Which may not be the look you're going for. Any kind of vent in the front of the hood will probably help with that.
But even that may not work incredibly well because of the angle of the Cobra radiator. Again, look at the Coupe. The Backdraft is horizontal, but the FFR is angled the wrong way.
Vents in the back of the hood are also less helpful than you might think - aerodynamically speaking. Leave the hood latches open and drive down the freeway. Does your hood lift? Mine doesn't. If your's doesn't either, that means there's not enough pressure under the hood to need venting.
Some people say the hood scoop doesn't do anything either. But the string test proves that it does. It doesn't do much, but some air does enter the scoop at speed.