I don't like to say anything negative but there a few things that I don't like. I liked your design better whan the angle iron was that, angle. It would be alot stronger with the angle leg that you cut off. As built there will be a tendency for a few things to happen. As weight is added, either static or dynamic, the slotted flat legs are going to have a tendency to turn into "S"s. Also the slots are going to want to work themselves open as the bolts want to work their way thru. As already stated there is a big possibility that when you hit a bump/hole when towing the whole axle will want to rotate and slide on the slots.
Please seriously consider welding the axle on solid after you find the proper placement.