Alluminum underdrive pullies will free up some hp and mpg. I had the BBK ones on my 97gt and as my first mod I swear I could feel the difference. I have never heard about swapping to an aluminum trans. I have heard of aluminum driveshaft, but I haven't ever seen a before and after dyno comparison to back up any claims. I'm pretty sure the case on both the auto and 5 speed are aluminum anyways. Yes a change in rear gears can help mpg. I recemond 3.73, or 4.10, some companies beside FRPP make a 3.90 as well. A custom dyno tune with a tuner will optimize your combo and give more efficient performance. A K&n stlye filter or complete cold air induction kit will also help.