Young Gun,
All transmissions (unless they have Overdrive) have a 1:1 high gear. This means your 3-speed and 4-speed will perform the same once in high gear. This is also true for the automatics like the TH350 and TH400. So 3000 engine rpm gives the same results with any of these 1:1 transmission. If you want to go faster you need to go with different gears in the rear, run taller rear tires or go with an overdrive transmission.
Your 4-speed should be the SM420 which is a bullet proof transmission, Chevrolet used it for almost 20 years. Unless there's something wrong with it I'd leave it in the truck and drive it like a 3-speed using 2, 3 & 4 only...that's the way I use mine.
Mike B
