Throw a 454 with a stock quadrajet in there and watch your mileage jump to 10-13 mpg. More power will make it nicer to drive and keep you out of the secondaries, thats why your truck is getting such poor mileage. Have you tried tuning your secondary weights?
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo