There are rear axles available with faster gears.
The simplest would be a single speed from the same years, they bolt in and can have marginally faster gears.
Here's a tech tip about swapping a later model rear axle with lots of faster gear choices.
http://www.stovebolt.com/techtips/bigbolts/p30swap/Only big complication is they have disc brakes, and it's normal practice to also have disc on the front if you want them on the rear, and putting them on the front is not nearly as easy as the rear axle swap.
Also don't be fooled into thinking you can travel as fast as you want with the proper gearing. The 261, while it is a good engine, with it's still relatively low HP in combination with the heavy truck will limit how fast you can go. Just guessing from my experience, but I expect you'd be unhappy with anything faster than 4.56 gears.
An overdrive transmission swap like the NV4500 is also a great idea.
Here's a site where you can calculate speeds and RPM with given ratios and tire sizes.
http://www.onlineconversion.com/bigger_tires.htmGrigg
