Actually... if you switch to a quadrajet 4-barrel(people call them quadra junk) it will actually get better gas mileage. The primaries(front 2 barrels) are much smaller than ur current 2 barrell. But when the secondaries open they are huge you will feel a swift kick in ur pants as opposed to the 2 barrell. you can get better gas mileage if you stay out of the 4 barrels. Headders will definitely improve gas mileage and horsepower as well. By increasing volumetric efficiency they improve horsepower and driveability as well. The only down side is that u'll have to find a 4 barrell manifold for the carb. and swap it. It can be very challenging to a first timer. Good luck!