[cleanspeed1]

Alright! Best B Series motor built, best block, easiest to modify, best injection pump ( Bosch P7100 ).

Service the truck front to back, convert to synthetics.

Run the hottest thermostat available and convert cooling system over to Evans Coolant.

Install a FASS or Airdog system.

Put the biggest open element air filter that can fit ( k&n or equivalent )

Decent sized aftermarket exhaust system from turbo to tailpipe.

TST Performance plate kit for the pump ( something mild, call for a recommendation )

Fresh set of injectors ( Dynomite, Scheid, any of the established shops )

Do the KDP fix ( Killer Dowel Pin, will save your engine )

Retorque the head ( I know it has nothing to do with mileage, but good preventive maintenance )

If the transmission hasn't been freshened up, get it done with a shift improver kit

Do a gear swap ( you didn't say if it was a 2WD or 4WD ), if you are not towing on a regular and if you can find a set, go to a 3.07 ring and pinion.

Your cruise rpm is 1600-1700 rpms.

Change your driving habits.

EDIT- MSD/Superchips Propane Injection Kit; will boost the mpg substantially with a nice power increase. Look online for the test that Diesel Power did on a Cummins powered Dodge Dually for that one.