Thanks for the photos, Tri5 and Webster and the rest of the input I've already gotten!

Crenwelge: "welding the web"...does that mean the inside channel of the frame?
When installing the C-channel on the frame, does it have a lip (measurement 'D') like this: http://www.johnsonrollforming.com/images/profiles/CC-1002.jpg
It seems the lip would make it harder to 'clip' onto the outside of the frame, unless the C-channel is slipped onto one section of the frame before welding the two together.
Or maybe the C-channel that'd be used for this job wouldn't have that lip?
Would you weld the C-channel to the frame...or is it flange-bolted to frame when you bolt in the crossmember?

Thanks much! This is seeming more like a possibility!

1958 Chevrolet Viking 60
Assortment of photos