Missed part one someplace...maybe this stuff was covered in part one...maybe not.

Anyway, look at what manufacturers are doing today to improve MPG. Fuel injection is obvious, the 92 Dak should have it. Leave the O2 sensor, let the computer control the fuel, it will do a better job of controlling fuel use at all RPM ranges than you could do. Low tension piston rings. Look at any gen3 hemi or LS engine with over 100K, you can sometimes still see cross hatch in the cylinders. That's low ring tension and fuel injection with no fuel wash. Look at eliminating other friction in the engine; roller cam/lifters/rockers. Let the engine breathe, nice big cold air intake. Free flowing exhaust includes headers.

You mention not looking at tires. While you don't want/need rubber band tires, you do need an smooth, even tread pattern, the big, knobby, mudder tires will kill MPG. With the trans, A-500 with lock-up converter and a reasonable rear gear ratio. Might have a lot of takers on that 518 trans you're taking out...might fund other things.

Think about driving habits. Using AC is better than having windows down. Using no AC and windows up is best. Trim the fat on the truck, low weight will help the cause. Fiberglass hood, bumpers, remove the tailgate for a net. Try to trim 200-400lbs from the truck. Funny how some of the MPG improvements are also things done on a race car.