Many, many moons ago I had the 8.3C in our 1994 Dynasty upgraded with a Bank's turbo kit. It was very early days so I had to go to their factory to have it installed. The upgrade raised the horse power from the original 300 to around 375 and the torque proportionately.
What is important here is that I'd had the Allison transmission repaired after a coolant leak into the system and at that time paid a little extra to have the gearbox upgraded from the MD spec to the HD spec to handle the extra torque the Bank's engine delivers. As I had effectively an HD transmission the Banks technician changed the transmission programming so I am able to get all the torque the upgraded engine can deliver, even in the lower gears! When in a hurry or climbing a grade I drive based upon the TIT (turbine intake temperature) gauge reading, backing off if the reading gets up to 1350F, the temperature I was advised beyond which the impeller blade can begin to deteriorate.
Since the upgrade we've traveled over 100,000 miles without any issues at all. Except four wheelers are often surprised when our coach beats them away from a stop light and proceeds to leave them behind unless they stand on their throttle.
My advice is to consider rebuilding your old engine and upgrading it to get the extra torque you desire. I think that would be the most cost effective solution. The Cummins engines are very reconfigurable using different cylinder sleeves, pistons and camshafts. An experienced Cummins technician should be able to rebuild your original engine to satisfy your requirements using just OEM replacement parts and avoid any issues with the electronics.
