I did mine the same way, but I cut all the way so the tube was flush with the floorboards (had to cut a few inches off the bottom of the doors which were smashed anyway). Mine are just welded to the body, but I took some 2x2x.120 and put in four supports total. I cut open the beam that connects the firewall body mount and the A pillar and reworked it with the 2x2. I ran the supports to the vertical plate that's approximately between your feet and runs from the firewall back to beginning of the bed.
I used 2x5x.188 for my slider tubes. The B pillar is almost 5" wide if you cut it flush to the floorboards, the A pillar is probably 4-4.5" wide there. I welded the open end of the pillars to the tube where they intersect. The tube can put very little bending moment on the pillars with the geometry there, and the floorboard is loaded in shear (especially with the 2x2 supports that I welded to the floor) if you were to push the slider tube inward toward the frame rail. And like I said, pushing upward on the outer or inner edge has very little bending moment because the pillars are so wide.
I don't see it being necessary to go to the frame in this case. I have intentionally abused the hell out of my sliders in the rocks with no issues. I have frequently dropped a 40" off a rock and slammed down on the center of the slider tube, hanging both tires on that side of the truck in the air. While hanging there on a rock, I can open and close the door with no issues.
This was before I was finished welding obviously. I kind of guessed as to how I'll be mounting racing seats eventually when I do a cage and tried to make the center supports convenient for that, but I didn't feel like interfering with the seats in there now.
I had some rust I cut out from the sills, this was before I filled in with 16 gauge.
the rear...
the front...
I'll say you told me so...
