Gen 2 V6's have a 5 speed automatic with 3.73 gears. V8's have a 6 speed automatic with 3.55 gears. EPA mpg ratings are the same for both I think, 14/19 IIRC. I have a cat-back exhaust and on the highway 19 is the least I'll get as long as I keep the speed down and often I'll get 21-22 with the cruise set on 65. If I'm running at 80 or so then it drops to 16-17. In town I get 14 at best and most every tank averages about 15 mpg. I reset the mpg thingy on the dash at every fill-up. Besides the exhaust I run synthetic oil, usually Mobil 1 0w-30, Mobil 1 ATF in the transfer case, and Mobil 1 lube in the rear axle.
If you get a decent deal on a used V8, go for it. I got mine new, the V8 option cost a lot of money, and I don't really need the extra power anyway.