[artdutra04]

As long as you use enough of them, rivets work just fine for holding frames together, especially since you also have the top and bottom gusset plates. For some of the more high strength areas, you may want to stick with 3/16" steel rivets (such as where the front/rear frame members intersect with the longitudinal drive train members), but for things like the top and bottom gusset plates, 1/8" steel rivets work just fine.

Unless you need something to be removable, use rivets if at all possible. It makes it so much easier when you don't have to figure out how to get a wrench into the chassis to tighten nuts. ;-)

Otherwise, this definitely looks like a good chassis and drive train design so far.