I'm not too certain about the 1948s, but I plan on doing the same thing with my 65 Stepside. I was fortunate to be able to find a 6-lug, 3.08 rear end (entire axle) for only 125 bucks. And it's a 12 bolt, coil spring!
I'll just swap out the whole axle and I'll be ready to go.
Maybe you can do the same thing for your '48. While you're at it, you might as well install new wheel bearings and wheel cylinders. Make sure your braking system is upgraded as well, as your truck will now be cruising at speeds well above what it was initially intended for.