Lets assume it is the Eaton axle and not a Dana (does it have a "third member" that pulls out from the pinion side?).
There is no lower ratio available for the Eaton axle to make it spin faster. It is stuck with the 4.10 gears for low speed. If you want a faster axle you must buy a different one. Consider the GM 14 bolt if you want to have a strong (i.e. 3/4+ ton) rear axle that is still GM. Lots of gears available (for $) for that axle and they can be found relatively cheap in junk yards. You can also put disk brakes on that axle. Lots of widths to chose from. Super strong, heavy axle. Make sure you get one with the carier that supports the lower ratio gears (4.57 and down I believe) and not the higher ratio gears (4.57 and up I believe).
If he's building a race truck, maybe get a Furd 9" rear, custom built. Lots of $'s but you'll get a good strong axle that is exactly the width you want.
If you chose to get rid of that Eaton axle, PLEASE!!!! put it for sale on this site. There are quite a few people (Denny are you listening) who would love to get there hands on that 4.10 Eaton gear to replace their 4.57 or 5.13 gear.
