You already have the highest gear ratio available for that housing, so gearing up is a pipe dream. Taller tires might help a little if something larger than what you have on the truck is available. The V8 will cruise at a little higher RPM, but it will be noisy. I ran a 59 Burb with a 307 engine, a SM420 trans, and a 3.73 gear ratio from California to Tennessee, heavily loaded and towing a small car, and averaged 10 MPG the whole 2500 miles. Your van won't come anywhere close to that MPG, even with the 350+ V8. Count on about 6 MPG on a good day with a tailwind. I'd choose to plan a route that takes advantage of as many secondary roads as possible, take some time, and enjoy the scenery going by at about 50 MPH as the 235 chugs along.
Jerry
