Straight 6 vs 305 TBI Fuel Mileage

[chevymotocross]

I got a new job, traveling pretty far, and I go through a tank of gas a week in my 87 r10 short bed. It has a 305 tbi, 700r4, 3.08 gears, and I'm only getting 13mpg. Mods include true dual exhaust, k&n filter, new fuel filter. It's mainly city driving. I have an opportunity to get a very clean 1976 c10 long bed with a 250 straight 6 auto for $1000. I wasn't really wanting to get a 2nd truck, I was looking into a small car for commuting, but since it's a square body with a 6, it caught my eye. Will a truck like this get any better mileage than a 305 TBI for city driving? Or, is it worth just getting a car.