You can use this site to help you figure gears.
http://www.drivetraindirect.com/t_gear_calculator.htm

For your towing I would not go any faster on the rear.

That leaves the question of how to get better mileage.
You may look in to an overdrive unit like the Gear Vendors, Ranger, or the Saturn. Probably in that order.
http://www.gearvendors.com/
http://www.high-impact.net/transmission_and_gear/rangeroverdrive.htm
http://72.20.96.178/commerce/ccc1105-overdrive-units.htm

They should solve your problem, and make towing even better.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-