Cholly- just what others said, you'll have to drill out the rivets (essentially, cutting away the head of the rivet). A Dremel tool helps greatly.
Keep in mind you'll need a way to reattach the chassis to the body.
You can also hand drill the rivet out, but that takes a while (and a good toll on your hand- at least it did mine.)
Somewhere on youtube are a couple videos showing how to do it. I wish I could find them here, now, for ya, but I have to head to work!
Edit: Did one more search- here's a video to start you with: