I boxed mine, using rectangular tubing inside the C channel. Pretty much *had* to; I moved the shackle hangers back for longer rear springs. The C channel was not strong enough.

My spare tire still wiggles on the rear bumper even with this. The rear frame is very flexy. If I had it to do over (and we always seem to do over) I would extend rectangular tubing up into the frame as far as possible, then extend it out the rear for a bumper mount. 'Twould be much stronger.

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell