Being done every day. We usually plasma cut the sides out of 11 gauge cold rolled flat stock and then mig weld the top and bottom 2" flat strap to the sides. This gives the deep center section and tapered front and rear ends much like the factory frames. One can also contour the side rails to fit the floor pan profile if it has been modified. An advantage over the tubing, other than looks, is that one can mark and drill the top strap, then weld on the "weld nuts" the same as the factory did on the original "C" channel frame. On tube frames many use Riv-Nuts. It is best to have a rigid table or jig to keep everything true. Using an "X" member or at least a "K" member will give a much stronger frame than the straight ladder type. That being said, we have done many with tubing which gives a smooth surface without hours of grinding and with a little planning can give nice suspension mount points for such things as ladder/four link mounts, Panard bars, etc. If you weld in anti-collapse tubes and use bolts that pass through both sides of the frame for the various components it's bukoo strong.

Evan