Why are you getting GT-40P headers? Do you have GT-40P heads?
I would get a set of long tube if I were you. They may be a bit of a pain to install, but the power you will gain will be worth it. Besides shorty headers aren't worth that much on a stock to mildy modded stangs. I would also go with a 2 1/2" off-road H-pipe.
Long tubes, a off-road H-pipe, and a cat-back should yeild around 30-40hp, IMO.
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1987 Buick T-type
1998 HD Electra Elide
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