Sliding 2X3 rect tubing into rear frame rail.

I put mine into the rails on both ends. what I did is drill the bottom of the rail 3/8 holes( can't remember exactly how many) to do rosette/plug welds. Trimmed the very front lip of the factory rail back so the connecter would sit flush. I peeled a section of the floor up and made a plate the width of the rail. Plug welded it to the frame connecter and welded to both sides of the rail. Plenty strong I can pick up any rail and that whole end raises up.
Disturbed Duster
Post 8 in my build thread first pic shows the rear, not very clear, but enough to get the idea