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Now for mods like you wish, it will be easier to build the car while the engine is out.
SD Headers were a good pick up, now heres the decision you need to make, how soon do you want to hit your 300hp goal? If you can wait, start collecting parts now and build the engine while its out. If you want the engine in now, since you already have headers, order yourself a 3.5 MPS, create your own intake, and some 605's and get the car tuned, at that point you will have a decently quick GTP.
MAKE SURE YOU SCAN THE CAR AT ALL TIMES, i read you have the Torque app but theres a few things you need to do before it reads KR. Otherwise purchase yourself an aeroforce scan gauge, monitoring for KR is a must when being modded.
If you want to hit 300+ whp your best bang for your buck would be a turbo kit. Youll hit higher then 300whp no problem.
But if you really want to stay supercharged, youll need to collect quite a few parts. Unless you just want 300chp which is pretty easy.
Read this, its the bible for our cars:
http://www.grandprixforums.net/safel...0-a-67580.html