350 mpg vs 454 mpg in stock form

[grancuda]

Looks like from all the stuff I have seen and read am going to scrap the idea of a 5.3L. I am now planning on going with a 350 but ran across a '96 model vortec 454 for $1000 that has 30k miles on it. Now I am considering a this motor but I don't know if it will work with the stock I6 cyl Saginaw 3 speed bellhousing/flywheel. So what mileage you guys getting with 2wd non OD stock built 350's & 454's. I am planning on a edelbrock airgap, edelbrock avs carb and headers and I am running a stick and 3.73 12 bolt rear. So with all you guys experiance what would my mileage be with 350 & with 454.