After getting the new rear frame rails exactly where I wanted them, I began designing the additional bracing to ensure they stayed in place.
In the back I welded in a 2x3 steel rectangular tube, (0.120 wall) and then I made triangulated braces for each side of the rails. The holes in the angled braces are to allow access to the rear bumper bracket bolts.
To securely attach the front end of the rear frame rails, I cut out sections of the floorboards, and used more 2x3 tubing to connect the rocker panels to the new sub-frame connectors. While I was at it, I used some 0.125 plate to connect the new torque boxes, sub-frames and rockers. You may be wondering why I'm doing this, but it will become clear soon.
The area with the yellow X got a cap welded on, so it is sealed to moisture and dirt. These added tubes are going to be the primary connection for my roll cage. Stay tuned.