choosing a gear ratio.

Ok guys,

looking at buying a used truck, this would be my first truck to lease back to my current carrier until it is paid off there is:
Prostar Eagle
cummins isx15 450 that can be opened to 475.
22.5 lo pro
FO-16E310C-LAS
1650 lbs. ft. [2237 Nm] 10 forward, 2 reverse
Transmission is an Ultra shift plus due to my disabilies. And lack of coordination.
You want the numbers from the LAS 10C line for any calculations.

I live in Washington and work out of Iowa so my primary lane is Seattle to Chicago and back hauling 35,000 to 44,000 in a reefer both ways.

I would like to cruise through WI, MN, SD, WY at 72mph but under 1350RPM, and still have enough torque to get up and down the mountains of WY MT ID WA with out holding up traffic too much currently the gear ratio is 3.36 Meritor axels. There are also some runs to the east coast on I-80 if I am not ready to go home.

Would anyone sugest changing it, and if so, Why?
Is it smart to turn up the HP on the engine or does that put something at a higher risk?
The truck has 540,000 miles on it getting 6.8 mpg.
Thanks for your input