Jerry when they class rapids a lot has to do with how difficult it would be to get help to you or you out to help if the need arises. A class 3 rapid in an easy to get to spot could be a class 5 rapid in a remote location. In the Grand Canyon There are 18 foot standing waves(they are always there). Eight of us on a 40 man raft cut it right in half. As for the tripod I would recommend a slinky. Not one of those cheap plastic ones though. One of the really good steel ones. You may want two for a 500mm. I believe that duct tape would be the mounting system of choice. You could dip the tops of a couple of oatmeal boxes in wax and push the slinky into the wax while still warm for feet and mount those of course with duct tape. For more information on how to properly use duct tape please watch this informative video.
http://www.youtube.com/watch?v=c5GnBvpFnzA
Good luck on your trip and we will be watching for some great photos upon your safe return.