Just to put this out there for everyone since I was getting questions on how I did the mounting:
The sub-structure has holes in it already. The feet on the factory rack have studs hanging down the bottom. Those studs go through the holes in the substructure, then there is a nut that clamps them down. That's it. Pretty simple.
In my case, I turned some aluminum rounds on the lathe to the right diameter, and then drilled and tapped the center. So they are aluminum "pucks" just over 1" diameter and maybe 1.5" tall. An 8mm bolt will go up through the sub-structure and into the pucks. That's how it's clamped. Those pucks are welded into the rack.
It's a bit tricky, because the substructure is dead flat. When they designed the truck, they made the pads where the rack sits dead flat. But the roof panel is not flat. The angle is different in every place. That's the trick. You need a way to seal the roof holes.
So what I did... this is confusing... I took the pucks, and bolted them down. Then I took a 2x2" piece of aluminum, and bored out the center so it slipped over the pucks. The plates sit flat on the roof so they can seal. The plates are at an angle to the pucks, since the pucks are level. With some fiberglass cloth under the plates as heat shielding, I tacked the plates to the pucks at the angle they rest at naturally when sitting on the roof.
Took that all off, and fully welded them. Finally, the tubing legs on the rack, are hand filed to sit flush on the plates which are sitting at weird angles because of the roof profile. Then, tack, and weld.
Difficult to describe in words. Impossible to manufacture at a profit and price-point that anybody would pay.