How much HP/Torque will this add?
 

I'm buying a 1989 Mustang GT Convertible.

Its stock, except:

Headers
Cold Air Intake
H Pipe

Stock its 225@4200rpm
300lbs@3200rpm

With those mods you are looking at 15-20 hp and 10-15 ft/lbs torque. With gears, underdrive pulleys and a mass air meter, you will really wake that car up.