I think you got it backwards there. Cummins recommends that you operate the
engine ABOVE the peak torque rpm (but below max operating rpm) in full
throttle conditions.

From the Cummins ISB owner's manual:

Do not operate the engine at full throttle operation below peak torque rpm
(refer to engine dataplate for peak torque rpm) for more than 30 seconds.
Operating the engine at full throttle below peak torque will shorten engine
life to overhaul, can cause serious engine damage, and is considered engine
abuse.
Cummins engines are designed to operate successfully at full throttle under
transient conditions down to peak torque engine speed. This is consistent
with recommended operating practices.

Do not operate the engine beyond the maximum engine speed. Operating the
engine beyond the maximum engine speed can cause severe engine damage. Use
proper operating techniques for the vehicle, vessel, or equipment to prevent
engine overspeed.

Check the dataplate on the front of your engine for the peak torque rpm and
the maximum operating rpm. When you are climbing hills you want to be in the
rpm range above peak torque but below maximum engine rpm. You can be at full
throttle for extended periods within this rpm range. If you are below the
torque peak rpm when climbing a hill at full throttle, downshift. You can
drive as fast as you want with overdrive locked out as long as you don't
exceed the maximum rpm.