Dave,
Why change the axle? I'm adding dual wheels to my '59 3800 9' pickup.
You'll just have to widen the rear fenders slightly (like about 12 inches!).
As Bruce said, good luck in finding a box. I lucked out and bought someone else's abandoned project for $ 600.00 (the '57 in my sig) that came with a box in great shape. The box off my '59 1/2 ton was toast. No one makes new box sides for the 9 foot box. All the other parts are identical to the shorter boxes, you just need more crossbraces. Even the wood and bed strips are available. You may be able to graft a couple of sides from a 1/2 ton together to make the 9 foot sides if you really want to.
Check out my pics at
http://community.webshots.com/user/Lane9999 if you want to see what I started with and where I am now.
Lane