You need turbos that are the right size for a Mustang, VW turbos are small, not even 2 of them will be enough.
Some pretty common turbos for Twin setups are from the SVOs, Saabs, and other cars that have at least half the displacement of a 5.0.
Single junkyard turbos are most commonly found in some trucks.
Check our Turbo-How to series:
http://www.allfordmustangs.com/AFM_T...on/index.shtml
Boost control: You use something called a BLOW OFF valve, which at a certain boost pressure will release some of the boost to the atmosphere. You can attach an electronic or manual boost controller to the valve, and this provides adjustabiliy at your fingertips.