Also keep in mind that you need to be able to quickly take the bumpers off and put them on (for inspection at minimum, if you have reversible bumpers or a bumper cover). The best way we've found so far:
Have a couple short sections of c-channel attached to the back of the bumper. These slide around the kit-chassis c-channel, and allow you to either drop a pin in to attach, or use a 1/4-20 to go through the whole thing. If using a 1/4-20, it's helpful to use a PEM nut or rivet nut on the bottom side so you don't have to get a wrench in there. Having a drill-powered Allen wrench to tighten the bolts makes the process quicker too. Of course, this assumes you can push the bumpers on from the side - if you're going with a single contiguous bumper that drops over the top, you can't use c-channel... maybe some L brackets?
Make sure you follow the bumper rules to the letter... it's amazing how much of the inspection checklist is taken up by the bumper rules!
Also, reversible bumpers are awesome. We used them last year... you can actually see us changing them while on the field at a pre-ship scrimmage:
http://www.chiefdelphi.com/media/photos/36403