I'd have to agree with John, your car can handle more than a 75 in a technical sense, but depending on it's history and for you own budget sake, hit it with a 75. Bump it higher if you really want to, just understand the consequences you might run into.
Here's the thing about hp on a cobra. I'm pretty sure the ratings for the engine and tranny are 400hp. Now you start with 305. Will they take more than 400? Yes but again how much more depends on your engine. If something goes wrong. It gets expensive REALLY fast. Through harris Ford in lynnwood (Which I don't ever suggest going to EVER). I was quoted $1500 just to remove the heads. That was labor only. Iwas also told by a couple places. That you can't machine the heads down. There is supposed to be not enough room in the chains for the camshafts. I would suggest a few more mods to make it faster. Than see where you are at. If you are ready to step up to nitrous. Then by all means go for it. What do you have already done to the car?