What is the average gas mileage for the 350 conversion?

You gotta be kidding me........

Actually, it depends on how many quarts of Slick-50 you intend to have in the crankcase and if you intend to install the Super Saver Gas Mileage Calibrator, Mark III device. However, if you install a large enough rear spoiler and get your self a decent size air scoop in the front this would definitely help. You might be better off, however, asking this question over at zcar.com, as that is where the experts are concerning this matter.