a throttle body on its own will not do much....
you can add about 15% plus increase in hp over stock with the following combination:
-cold air intake AND 73mm throttle body
-performance exhaust, catbacks / x-pipe
-re tune of engine management system with tuner (diablo predator or equivalent)
I believe they come with a 65mm or 70mm......i think its a 65mm.
Goto an autoparts website and look up Throttle Body for your mustang.....it will say the mm size.
BTW, DONT even bother upgrading to a bigger throttle body, this has been PROVEN a complete waste of money!
(think about it for a second...why get a bigger throttle body? That DOESNT change the size or flow patterns of the Intake Manifold, so it's kind of like increasing a pipe diameter, then squeezing it back down again, its truely pointless!
Goto ANY mustang message boards, they will confirm.
If you want to benefit from a larger Throttle Body, you MUST atleast Port your Intake Manifolds and Cylinder Heads first!
(which is alot of money....or alot of work if doing it yourself.)
I would go with a 70mm BBK Throttle Body....some other good parts you can get to improve performance would include Cold Air Intake (good brands:JLT, BBK, Steeda), Underdrive pullies (good brands BBK, Steeda), Exhaust (brands way too many good ones)....check out www.mustangunlimited.com and www.summitracing.com for the best pricing on these items...GL!
Mike is right.A Throttle body on a pretty much stock car is basically useless unless you are putting down some serious power n/a or you have forced induction you really wont benefit from a throttle body.