[cwl1979]

I'm no expert on this but i don't believe that would be a good idea. While the 5.9 Cummins is an amazing engine, its much smaller than what you'd be replacing. That old 555 is just that, a large V-8 with 555 cubic inches, the 5.9 is only 360. Id be worried about overheating and overworking it on the daily. Thinner cylinder walls and lowered cooling capacity. If it were me looking to "upgrade" on a repower id probably start looking for a Cummins 400 big cam. You'll probably have to fab new motor mounts either way since you'd be going from a V-8 to an inline 6. Perhaps look for an 8v-92?

As for the transmission you're looking at, its a solid transmission. Coming from a military transport background, its tested and proven.