Really don't think it's going to do you much good, and would be a lot of work for minimal to zero improvement. You'll improve mileage by drastically reducing weight, improving engine efficiency, modifying gear ratios, decreasing rolling resistance, improving aerodynamic qualities of the body...
Pulling two of the rear wheels and tires off will net you around 150 pounds or less weight loss; you'd still be running the heavy full floater rear in a single wheel setup. The change from a dually style front wheel to single wheel style only drops you a little weight from the shorter hubs. Then once you pull the dually flares off the bed and install a bed that isn't cut up, maybe total you'd save 300 pounds? Compare that to total weight of the truck.
I'd say, get the engine in the best state of tune you can get it, keep the tires aired up to their max safe amount, if the tires need replacing consider whatever skinny tire might fit your wheel size and whether skinny tires would work for the way you want to use it...and just enjoy it as-is.
Welcome!
Richard