Diesel Bombers - Just pulled the trigger on a 2002 2500 LB7!

As you all can see I'm new here, and I hope I'm posting this in the right place, please let me know if I screwed up. I just purchased a very nice 2002 Duramax 2500 HD, Allison tranny, extended cab, 4x4 (it has the hand shifter on the floor, which surprised me, I thought they were supposed to be a push button???), it has a sweet aftermarket flatbed, with headache rack, and I just love the truck! It has 235,000 miles on her, just had her first set of injectors replaced (all of them), 2 new batteries, and the engine almost looks like the day it was made! Everything works, including the Tow mode on the trans, A/C, heater, etc. all work great too. I really drove the heck out of it, and it performed beautifully! It has a 4 or 5 inch exhaust from the turbo back, no muffler or cat, and a chrome tip. Other than that it's bone stock, and has never been chipped, or modded, besides the exhaust. I paid $6950.00 for the truck, and it has no rust, dents, or accidents, so I think I did ok with the price, even being stock. I'm only after the best fuel economy I can possibly get (and overall longevity of the drivetrain), as I do a lot of driving, and am wondering what things would be the cheapest to buy, to that end? I've read about the EFI live, and other programmers, down tubes, etc., but again, with only being concerned with mileage, what would be your expert suggestions? I will be hauling some small square bales of hay, and my trencher, on occasion, my dozer is way to big to haul with this truck, but I will have to see just how much towing, as only time will tell. Thanks in advance for your time and consideration!
Proud new LB7 Duramax owner! :c: