Well there is more than one problem with your car. Everything is completely mismiatched.
Do you know what kind of 351w you have? Is it a roller block? What compression ratio, it could be an older low compression 351. The stock heads are killing you on that big a motor. 4" Tips are unncessary, and the 2.73's are crap. The 30lb injectors are wayy to big and making your car go very rich I'm sure. Probalby the biggest source of your low power. Not to mention if the cam you have doesn't match your stock heads. Also what is your timing at?
Go with a set of TW heads, fine out about your short block and cam. Do your research before you mod your car, as the person that had it before you didn't.
Good luck...
Skyler
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-1989 Saleen Mustang #406- TFS Heads, E-303, edelbrock intake,70MM TB, 73mm MAF, off road H, headers and 3chamber flows.
12.55@107mph
50-175 Nitrous Works WET Kit just added.