I believe the cobra crank is what most people upgrade to. From what I've heard, the bottom end needs to be beefed up as well. Someone correct me if I'm wrong.
Not necessary but good insurance. I haven't heard of anyone breaking a GT crank yet (may have happened, I just haven't heard of it). Most people are just putting the cobra crank in when they build a new shortblock these days instead of wondering if the GT crank will hold when they decide to up the power more than they intended in the beginning.
I don't have a stud girdle on my car and it wasn't recommended to me so I'd pass on that.
As far as where to buy the cobra crank I couldn't tell you the cheapest place unless T&J is the cheapest which is where all my stuff comes from. Might try them. www.modularfordpowerhouse.com