Yeah the gears are gonna be a nice chuck of change. Maybe 2500 to 3000 if you already have limited slip and just need the gears. I went from 3.55 to 4.10 with stock tires and lost about .5 mpg overall. Closer to 2 mpg on the highway. 0 to 60 it pulls much better but I dont think I gained much in the 1/4 mile because it shifts into another gear. Also its hard to get traction.
You have a good plan with your mod list: intake, TB, muffler delete and hemifever. Maybe wait for a set of longtube headers instead of the gear change. With the custom tune you would get a good gain from the longtube headers.
Torque converter. I had a 3000 stall that was to high, I now have 2600 stall and think it is perfect for my truck. Stick to an 11 inch TC.