Big46, I'll throw in with Tony (Truckedup) on this one. I am not intent upon raining on your parade, but make sure you have a safe, enjoyable parade.
California freeways are nuthin' to mess with--little bitty ricers goin' 80 mph flittin' around monster grocery carts (Hummers, Escalades, Excursions, and such preferred chariots for modern shoppers) doin' 75--in an old truck designed and outfitted (as Tony says) for 45 mph.
If you stick different gears in the rear end to increase angular momentum of the tire at the tire-road surface interface (that makes it go faster--sorta like a new paint job!), you necessarily change all the parameters viz. a viz. the acceleration (but more importantly, deceleration) equation, not to mention steering, mirrors, stop lights, ad nauseam.
I would highly recommend you do something before modifications that affect the coefficient of friction between the brake pads and the drums, to beef up the braking power delivery system (that Rube Goldberg bunch of pedal-levers-foot long cylinder-fluid tubes-wheel cylinders-expansive lever actuators-spring loaded pads). Else, it is but a matter of time until you will find yourself with a little fly in front of you that you could stomp on, or a unit transportation delivery vehicle larger than my Grandad's house, doing wierd things in front of you. Por supuesto you will stomp down on the brake pedal, maybe with both feet (who gives a #@%&@ about the clutch at a moment like this!), which will take up an inordinate amount of your 1 to 2 second reaction time before you factor in the reaction time of Rube's contraptions, and that miniscule delay at 70 per can easily account for 100 or more feet of roadway you just kissed "goodby" even before your precious truck begins turning angular tire momentum into dissipated heat energy.
Stand on the brakes too much, and you will literally "burn rubber" (or whatever synthetic concoction your tires purport to be) to transform the kinetic energy of a loaded single unit truck into expended waste.
Single unit trucks, loaded, are to be respected--both as objects of great admiration and for what they can do for us as well as to us.
