Just out of curiosity, what's wrong the original axle? Just needs refreshing, or is there some sort of parts failure, like damaged gears or broken axle shafts? The stock axle from the '55-'62 trucks is very strong, the Gassers from the '60s (like Stone, Woods, & Cooks Swindler Willys) used them with well over 800 hp regularly. As far as gearing, you most liley have the 3.90 ggear, which works very well with an overdrive trans. I just had mine out on the highway not too long ago, ran 75-80 mph all day long. I'm running the original style Borg-Warner overdrive, which is a 3 speed with an auxiliary trans, but the T5 would give you similar results. About the only 4 speed in the game that would help on the gear ratios would be the MY6, which was a Chevrolet version of the Chrysler A833 4 speed. Used mostly in early '80s 1/2 tons. Good trans, and should fit in your truck nicely. I do beleive there might be some alterations needed to the front bearing retainer for it to fit. You might try searching "T5 swap" and "MY6 transmission", should be able to get the exact information.
