Arne, good point. I am going to use a simple plate as suggested. My goal was to transfer load from the end of the inner tongue channel to the side rails.
Thanks again everyone I now have a plan for the frame.

It will be light and I'm going to trust the cabin box for strength. If it fails I'll fix it, but outside of the tongue I can't find where anyone has had a major frame failure. (I'm sure there has been but it seems to be rare.) As I research all the different materials, skills, knowledge and designs people use, I find that they all seem to work. This would not be the case if we were building utility trailers I’m sure. So IMHO, outside of the tongue and securing your running gear, everything else is tie down points. Or what Grant said...
But the bottom line is that a properly built teardrop body is a rigid torsional box ... you could build the under-body part of the chassis out of bed rails and you'd be O.K., as that part of the frame is only there to mount the axle and the frame tongue. The critical parts of the chassis are the tongue itself and how it's tied to the under-body framework.
Great forum and thanks for the interesting discussion. Now I have to figure out the floor / frame / curved body / and aluminum relationship at the front of the trailer.
Rodger