Yeah, you want clamps on there. Take a look at a good aftermarket spring pack. They use a clamp, not only to keep them in-line, but to keep them from spreading too bad. The spreading is usually limited to ~1/2" or so, after that, the clamp transfers the weight of the axle to the top of the main leaf. Some spreading is ok, as long as there is a clamp to transfer the load.
A traction bar is to cure axle wrap, and won't help leaf spreading. Without clamps on there, your going to get some wrap, especially when the axle is drooped, but once you fix the clamps you'll be fine.
And yes, weld that wheel well!