Mike,
I was thinking about a welded frame (5x9) and reworked your drawing (I used Photoshop, so it's not quite right). Anyway, do you think that I would need to box the front or second crossmember (or both) to add for the welds? Or would the angle be enough. I was also thinking that maybe the crossmembers should be cut in for the square tubing of the tongue, like Bufordt suggested.
Kerry