Instead of taking the time to box it in, just run two diagonal braces from the reciever up to the bottom or sides of the frame. I weld pretty much everything on, but there's no reason you couldn't bolt it if you wanted to.

I'm missing several inches of rear frame (12" bed bob) and the bumper is a bit, er, dfferent than a stocker, but this design works very well. Yanked with it and been winched until I was pretty much hanging from that rear bumper tow point, and it's worked great so far.

Here's a pic from 2003, 3 rollovers and a lot of dents and cage work have happened, but it shows the rear bumper I've done. If you don't still have a spare tire down there, this is probably the strongest way to go: