Listen here '68gem, There is a lot of good carb theory goin' on here, and you should by all means absorb it and learn it. I don't know the extent of your expertise, that's why I say this. Most of what's bein' said here is true. I don't know about the only 2 sizes of Q-jet thing. I could've sworn my SBC 400's was what they considered 835 CFM, according to the research and books that I've read, but that's another post. If I were you I would keep the stocker and rebuild it. You're probably not going to get better than that for your mileage. I'm also gonna get a liitle off your original question. I'm guessing this is on your '68 3/4 ton truck? You've got a lot to think of beside carburation for your mileage on that tank. A 3/4 ton truck is goin' to be built heavier, stronger, and geared differently. My first guess would be that (if it's runnin' right) it could pull a stump out of the ground, or climb a tree. You shouldn't expect too much from what you're drivin' on the mileage side. That truck is probably geared way down from the factory. You can get better mileage, but don't expect much. I've seen some that do all right, but I wouldn't expect much more than 12 MPG out of a 3/4 ton truck, especially one of that year.

Like I said listen to what is being said here, there is some good theory bein' explained to you. Some good formulas that you can use for future reference as far as carburation goes, but look at the whole picture, not kust one aspect. Mileage is a combination of many things, including carbs, gears, cams, exhaust, etc..