I am assuming that the engine is pretty much stock and if it's not that it has just minor stuff. Get a Walbro 190 lph fuel pump. No it will not severely effect your gas mileage. Don't be talked into some expensive brand because most likely it is a Walbro with a "" sticker put on it.
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"Gripping the wheel his knuckles went white with desire, the wheels of his mustang exploding on the highway like a slug from a .45. True death. 400 horsepower of maximum performance piercing the night, this is Black Sunshine"