If you're going to have the injectors and pump reworked, why not also do the engine?
If you're keeping the 4.56:1 final drive, and 37" tires, you're going to have difficulty putting that truck on a freeway unless you get an over drive transmission - a 1:1 (direct) top gear
will get you ~68MPH, but I think you'll find that 2800RPM is going to SCREAM (if you thought a CUCV with no cab insulation was loud with a 6.2, just wait for it...
). You'll also be out of the power-band, so any need to pass is going to leave you wanting. Redline for the DT466 is 2800RPM, so if you're pushing that constantly, you should definitely check the crank balance and piston/rods for balance/matching - lest the vengeful gods of diesel propulsion see fit to smite thine oil pan with a thunderous piston of terror and piercing rod of doom...
Getting an overdrive of 0.78:1 will take 65MPH down to 2100RPM, an overdrive of 0.73:1 will take 65MPH down to 1950RPM. These are better revs IMHO, I'm just thinking of redline at 2800RPM and the first downhill with a load that takes you past it...
The NP205 input torque rating is 1100-ftlbs, it should be fine with your engine selection, just check your bearings and seals.
Have you given thought to auto or manual?