Sorry it's taken me so long to reply here, been out and about. I made the rails out of 1/8" thick, 1-1/2" x 3" rectangular tube. I cut the ends at an angle and sealed 'em off and then welded a 1/8" thick, 1-1/2" strap full length along the back. The strap sticks up 1/2" above the top of the rail.
I then held the whole mess up under the van with the welded strap up against the body, and on the outside of where the inner and outer body panels come together, and drilled 5/16" holes thru the strap and double thick body panels. Make sure that you don't drill where there are any drain holes. I reused the two bolts that hold on the hoop steps and added nine others. I was going to weld several 3/4" x 3/4" tubes to the back of the rails and then fasten them to the frame. This really wouldn't make the rails hold any more weight, but it would resist the bending moment that would come from a vertical load right at the outside of the rails and also help with a side impact load. I've done this on my Bronco's and Jeep, but those rigs really took a beating. I figured the SMB will go nasty places but not really any hard core rock crawling so I left 'em off of this rig. Besides, I have plans for the open space between the inner body and the frame rail on the drivers side and didn't want to block off part of that. Anyway, the frame brackets would be easy to add if you want 'em.
As I mentioned, I can jack up the van with the rails the way they are and that should be good enough (yeah right) for me. Another reason I wanted the rails was to protect the rocker panels from flying rocks off the tires from high speed dirt driving and to hold up the side door stairs we attach when camping. (That's what the four little tabs on the passenger rail are for.
Anyway, here's some photos.
This is the passenger side rail
The driver's side
Front of rail
A wider rail could be used as a step and would protect the body more, but not what I wanted. Also, if you did go with a wider tube, the frame brackets wouldn't be a bad idea.
Anyway, I hope you guys can follow this. If not, let me know and I can take more pictures when I get 'em back from powder coating or I can draw up a little sketch.