Jim that is the older m 818 series they had non top stop injectors and a 250 cummins. an optional but rare motor was a turbocharged mack. I have driven them all the mack was a stump puller ,the cummins drove the best on the road and the 8.3's were the most efficient plus quite a bit lighter I saw factory 8.3's in asv's rated to 350 horses. I don't know if the 5 ton t-case could take it, as even the 8.3's were capable of cracking the t-cases in to 2 pieces if abused, say like rocking the truck to get unstuck. lastly I when stuck on a dyno the larger 855 will have more torque as the ratings were diferent back when the 855 was king I used to work at a military rebuild shop we had a dyno and every engine went on a dyno for an hour before we crated them up and the 855'5 always made better low end grunt. I'm not belittiling the 8.3 it's an impressive motor I would love to have one for my nodwell project, but the 855's had a better track record for reliability, except for cold starting. I should also add there was never a problem with the c series engines in the mechanical department it was almost always a fuel problem, either a fuel solenoid which were prone to burning up or the plumbing between the lift pump and tanks leaking air. both could be an easy fix using areoquip fittings and hose for your supply system and a cable shut down system the military wouldn't let us modify them with out authorization.