Conventional wisdom says that the 7.5" rear end in your car is good to about 300 HP. If you plan to build above that level, or if the car will see hard launches on the drag strip (higher gears and sticky tires), you should upgrade to the 8.8".
Getting an axle assembly from a GT will get you 3.73 gears, an 8.8 diff, and traction lock all at once. That will set you up with all you need.