Carbruator for best gas mileage

[Ricos54]

I have a stock 350 crate motor 250hp with a 700R4. I'm running 300 gears posi 9" and going to put 20's all the way around tires will be 255/45/20 in my 54 Chevy 1/2 ton truck. I'm trying to decide what would be the best Carbruator to give me the best gas mileage. I got a 670 4 barrel duel feed Holley street avenger. Holley said that's to big to go with there 570 street avenger. Edelbrock said to go with there Edelbrock performer series 600. I wouldn't mine have a little power, but over all I want good MPG.
I'm not building a hot rod, I'm a nice street rod that I can drive everyday and take trips in. I don't have a heavy foot and would like to see low 20's if possible on the HWY.
Also I'm building this truck on a budget so I don't have a bunch of money to throw at it, if I did I would have bought a LS motor with EFI. With that said what would work best for what I want?

Thanks