Sounds like you have a few spans of "free space" behind the bumper, doesn't it?
See if there's a way to run one horizontal frame member (or steel support piece) all along behind the bumper, about midway up it.
We designed our frame to have it's upper structural piece made as a big rectangle of squre tubing, at the middle of the bumper zone. We'll have brackets at the corners that go up above the frame, to allow the bumper bolts to attach.
http://www.chiefdelphi.com/forums/sh...5&postcount=28