If I were you, I would put plywood gussets on both sides of each corner and joint, like in this picture.
As for the axle, do I recall that you are planning a single drive wheel? If that is the case, you can get a pair of cold rolled steel rods that are the same size as your current axle. Then you can make a box like the one in the picture below to mount your new axles in. You should be able to pull it off without welding anything. I am not going to suggest anything that requires welding, but keep in mind that you need to make sure that you 1) keep the speed way down, and 2) check your bolts before every ride.