basically thats what you do when you modify one to get faster speeds out of it is use donor parts truck. you change engines, trans, diffs,transfer cases, ect. you would be ahead to find a donor truck if you want to go faster on the highway. with the 715/724/725/726's they werent made to go fast just go anywhere and carry heavy loads doing it. you will then lose the thing that makes these trucks special in that you lose that rock crawling, stump pulling, low geared ability that these trucks were made for. you can change the tranny to an overdrive and then you would need to change the transfer case as they heat up at speed and from what the guys say get extremely hot. then you would be able to keep your low gears and still get highway speeds. basically its what ever you plan on using it for that determines what you do with it, stock or modified, fast or slow speeds, ect.