What kind of bumper are you building? Tube?
If you are doing a tube bumper, just tie it into the frame on the side the same way you'd do a front prerunner style bumper. You want to do this for side impacts no matter what else you do.
Honestly, though, you'll have enough clearance back there that you shouldn't be hammering it. If you have plans for an internal cage, which you should if you really need to be cutting your quarter panels, then I'd just plate the folded portion with some 3/16" stock and brace it up into the cage from the inside, and skip trying to get bumper coverage, which, in the end, only costs you the clearance you were after in the first place.
I've been looking into the same thing, but I'm not sure I want to change my setup. I have the Tomken '97+ bumper that reuses the stock end caps. The plastic just bends up when I hit it, sometimes denting the quarter panel a bit, and then springs back into place. But I'd like a tube bumper to get the weight down back there, so I'll probably mess with it at some point. If I do, it will definitely tie into the cage.
Nay