trans and gear selections

[irishfan77]

I hope someone can help me out here. I have a run of the mill 350 chosen for my truck. I want to go fast when I want, but be able to do decent gas mileage on trips to shows. what gears in my rear should I use with a 700r4 trans as it is the o/d trans I plan to use. I am running 18's in front and 20's in the rear if that helps any. Any advice you have is helpful like better trans selection or gear sets. thanks!!