MPG's are just part of the story. If that 428 has not been rebuilt since '86, it is very likely on its' last legs. Typical diesel will be good for 4-500K miles with reasonable maintenance.
One big difference is torque. That's what moves mass. A gasser will have to work a WHOLE lot harder than a diesel to move the same load due to the large difference in the toque each puts out. Cubic inch for cubic inch, a diesel will create much more torque.
My little four cylinder Cummins currently makes a little over 350 pounds of torque. The eight cylinder 428 makes 390. Mildly tuned 6 banger Cummins routinely put out 600 pounds and can get 12-14 mpg.
There are lots of very good reasons virtually every large, commercial vehicle on the road is diesel powered.