I have 391's with an Eaton TruTrac from Dr. Diff. The Tremec trans has a low 3.27 first, and fourth runs 112 MPH at about 6000 RPM through the traps. I also took a suggestion from Rich Ehrinberg and mounted a set of kicker shocks horizontally to the axle. They dampen windup from launching and hard braking without constricting travel. It makes a versatile set up on a budget.