There's a return-less fuel system in your 1999 GT, You need to make sure the pump is made to work with a FPDM, (which equals to Variable Voltage out-put to the pump) The FPDM is going to controlled the fuel pump pressure to your engine true your cars ECU
Your Dyno MUST be on Steriods, No way in hell that Cobra is going to make all that power, with a port and cams and a HEATON, let's put it on a Dyno-Jet and see, I can even find U a Mustang dyno to put it on if U like!!!
Steeda always puts together a Bad A$$ Stang, they do a shit load of research and development on there after market Performance part for our Mustangs, I love this car, very nicely done, good find Mike