Your other problem with a detroit 2 stroke is that they make no power on the bottom end. If you drop em under about 1200 rpm under a load, you have a good chance of sucking the seals outta the blower as well. We have a old international dump truck at the mine with a 6-53, and a huber road grader with a 6v72. There is a reason they call em the screamin jimmy. Ya gotta keep em wound tight to make that power.
As far as baby motors go, you might check into a John Deere unit off a small generator. We have a couple of those as well. Prolly only about 200 horse or so, but both of our gen units at the mine are now over 20K hours with only a water pump replacement and regular oil changes every 300-350 hours.
The Cat 3126 is a good motor, but they are low power, high torque motors. They work great in the 4500 and above series trucks, but you'd have to check the weight to see if your front axle can support it. Probably about the same problem you'd have with swapping in a 8.3 cummins.
Regardless, if you can get adaptors made to work, you would definately have a heck of a one-off truck with any of those motors. I like the 5.9 cummins because of parts availability and it has a very wide powerband for a diesel.
