I've heard that if everything else is stock, then you should probably go with no more than 8psi. Possible damage could ensue otherwise. I have no data to back this up, so I may just be smoking some bad crack.
Not entirely sure. Generally speaking, parts are cheap. It's the labor that's expensive. I've found some 3.73 gears for $150, but people want 400+ to install them.
I'm in no way an expert on the mustang. I know there are a lot of things that can be done to decrease track times. I'm sure you've researched a lot of them. Personally, I like to go one mod at a time and get used to it using the butt-o-meter, and the 3.73's are my next thing.