Mopar claimed about 10% better miledge when they came out with the hemi, of course the trucks got huge-er at the same time. The wind drag from being so huge is going to hurt MPG as the speed goes up. I would not worry about power with the 3.21 gear as they dropped 1st gear ratio a lot also so take take off TQ will still be more you just get a better final drive ratio by having the higher rear gear.

One time I re-built one of those 5.7s for an 04 ram that coughed out a rod for no reason and we degreed the cam (stock cam we degreed mostly out of curiosity)and decided we should advance it one tooth and holy moly throttle response was waaaay better after that! I do not remember the numbers we came up with but figured it could only help and boy did it, the thing is now hyper responsive. We did end up with a CEL for cam/crank correlation or something similar but that truck is very responsive and quick for a 4 door 4x4. That was pretty much the only mod on the truck besides a crappy dual exhaust installed by previous owner. Really makes me want to put my 5.7 in my dakota but I already lose sleep worrying about the wiring.