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
'Sweet Spot' cruising speed as is, with ther 235, seems to be about 45 Miles per hour.
It will do more of course, but, you can feel it is not liking pushing all that Wind!
