If you want to keep up with that Cobra, you're going to HAVE to buy a supercharger on your Stang, either that, or you're going to be blowing so much Nitrous Oxide that you'll need a bottle the size of Texas to keep you going.
I don't think you'll need to step-up to the expense of an intercooled supercharger, a regular lower-pressure (7-8 psi) model should do the trick nicely, and put you right around 350-380 HP depending on the tune. If you go with an intercooled model, you can jack the boost up to 10-11 psi safely and be pushing 400-420 HP. That would get the attention of just about anybody.
Keep in mind, that for your supercharger system to really work well, you're also going to need some exhaust work. At least an H-pipe and cat-back muffler setup, and headers wouldn't hurt the situation. Total cost for your project should be in the $4000-$6000 range depending on how much you decide to go with .
Greg