I've reread this many times. I had a '06 AMI 370 4.17s and a Fuller 9 speed (overgear is pretty close as your transmission). I speced it to be light and aiming to get good fuel mileage. I can honestly say that was by far the worst truck I had ever purchased new. It was terrible on fuel, and by terrible I mean in the low 70s (weight) you had to drive it likean old lady to get in the four miles per gallon range. The thing was like a turtle on it's back. There were spots on blacktop it had all it could do to get moving. It was back to several dealers and on the dyno and ..... The truck was junk, and while a little worse than others , not night and day difference. Like I said, I've read this over a couple times, and from your first post ? I think I would leave it alone and keep doing what you're doing to it as far as operating it ( you're doing something right ) Those engines were filthy too, you go spending a bundle on reratioing it and next thing you know there's a hole in the block where a wrist pin let go. Personally, I think it's a case of "if it ain't broke.... don't fix it" The gearing it faster for mileage might not work on this one without spending a fortune.