the trans that I recommended before have a 1 to1 final drive ratio. If you are looking to reduce engine rpm it will not do it. the rear axle ratios gm used in your truck were 4.10:1 or 4.55:1 .The late 55 used a 3.90 ,58-59 had options of 3.70 and 3.38 ratios.Chevy used 6 lug wheels on 1/2 tons up to 1970,check width of axles to make sure they stay inside fenders,SUMMIT has pads to mate axle to springs if axle has coil springs.