A D400 is a pretty big truck that can carry a big load. If you intend to load it up, it might get pretty rough on the 727. Most modern big trucks with an auto trans has an Allison, and they are a lot more heavy duty then a 727.
The NP435 has a low 1st gear to get things moving from a dead stop.

My concern with the 727 would be about the life expectancy. Be sure its build for pulling a heavy truck with a heavy load, unless you don't intend to load the truck very heavily. Gene