Well to give you some background before i start dropping facts on you, the general rule with throttle bodies is:
1)65mm if you're planning to stay NA with a bolt on car.
2)70mm or higher if you plan on boosting it. from there on out it really depends on what size charger or turbo you are running.
Some guys have bought 70 mm TB's on basically stock bolt on cars and have noticed increased power AND torque. Others have noticed a loss in torque. It really depends on what kind of ohter intake mods you have done to your car as well as better breathing intakes and heads. All are factors.
The opening on the intake manifold is 70 mm in diameter so the air has a nice clean striaght through flow. Even though this might still be a little big depending on mods, its not overkill and may work well.
As far as your TB, i am sorry to tell you, but its wayyyyyyyyy too big. A 75 mm on a basically stock car will probably gain you a few whp but it will lose a good amount of torque. I say sell it and look for either a 70 or 65.