I would say 120 should be the upper limit on a 1955. The 1755 and 1855 do not have some of the reinforcements that the 1955 does, namely the deflection control bearing that runs against the back side of the differential ring gear. If you look at what left the factory, 110 is about the most they officially put through that transmission without adding outboard planetary final drives.

The 310 had good power, torque, and were quite economical on fuel as well. If they are built right, they start really good too. I am storing a 1755 for a friend, and I decided to see how good it would start in the winter. It was below freezing, no starting aids or preheating, and it fired up like it was 70 degrees. It is supposed to only have a few hundred hours on an overhaul, but whoever did it got the valve protrusion set right.

 Don't get me wrong on the Cummins engine, the B series are about as good a diesel engine as you could ask for.