Your mileage will drop considerably with MAYBE 19 being your high, although it being a carbed 4 speed 350, that is probably too hopeful of a number, especially with a larger tire. Realistically, I would be looking anywhere between 12/16. Good luck finding what you're looking for! That's not a bad price for what it is, but I would personally try to find a fuel injected one.
2005 Chevy Silverado Base 4X4: 4.8L, Auto, 4 door ex cab, short box, AEM Brute Force CAI, Flowmaster 40 SI/DO, 285's on 17 inch Helo's Summer and 265's on 17 inch Devino's for winter, custom made center console, custom engine tunes, custom headache rack, Kobalt contractors tool box, color matched grille and sport mirrors, HD towing mirrors, no power options but I got A/C!
1992 Ford Ranger XLT 4X4: Needs a lot more done to it!