This gear thing has been rung out on one of the other forums, sorry I did not pick up on this thread last month. Decades back the smart owner ops geared their rigs for cruising speed with the transmissions at 1:1 and save the overdrive(s) for faster and/or easy going situations, long gradual downhills like we have out west, etc. 1:1, the shaft going straight thru the tranny, saved on gear whine, countershaft gear loss inefficiencies and bearing wear.
With 22.5 tires and your trany in 1:1 (9th) a 2.5 rear would get you 65mph at 1400rpm, but don't know if can get a 2.5:1 gear set. 2.64 is available and it would get you about 63mph at 1400rpm and 67 at 1500rpm. Drop it into 10th and you'd get 66.5mph at 1100rpm, 73 at 1200, 79 at 1300 and 85mph at 1400rpm, etc.
Why do we need our highway cruise to be in the topmost gear with all the other gears stacked up down below? Put one of those gears above for those situations still leaves you 8 gears down below, plenty for our loads and lets you cruise with tranny in 1:1. Worth considering?