It might be helpful to get a more accurate answer if you would tell us what all of your combo is NOW. You know what car you have and how it's built now. We don't.
Mild build LC 340 with around 330hp, 3.23 gears (gear is because I do a decent amount of highway cruising) and standard 727. If I got 3.91 gears in the back along with the 5 speed, how much of a difference would that make?