In your transmission 16 gear would be direct drive, 17 gear would be your first overdrive, and 18 gear would be your second overdrive. With your set up, I would run 2:64 gears and mostly run in 16 gear. If you need to go faster you will then have 17th gear. 18th gear is just to fast to be a useful gear. Your Rpm may be closer to 1450 at 65mph in 16 gear. If you want it down to 1350 maybe see if there is a gear ratio available in the 2:50s. But I think a 2:64 would be a huge improvement over what you have and make your truck much more drivable. 3:91 is geared way to low for 65mph with that engine.
Chicotransport.inc will be eternally grateful.