It's a Ford 9", so another approach, if you happen to have a nice one in the stash from another kit (like a lot of Nascar race-cars, or the Revell '32 Ford kits) is to remove the diff from the springs and substitute the better one in its place.
Measure the width of the springs first, then cut the axle housings flush with the springs on either side. Take a round file and remove the part of the axle housings left in the saddles, leaving a nice half-round for the new axle housing to snuggle into. Glue it back in the same place, making sure to keep the spacing between the springs the same, and keep the pinion angle the same as well. Swap in the driveshaft and you're done.