I may have an extra Velorex-style U-clamp kicking around somewhere that you could clamp to your downtube to replace that threaded rod. Probably not the /best/ way to do it, but I think it would hold a lot better than how it is now. Although I 'spect you could achieve the same thing with just an eye bolt and a couple lengthwise halves of pipe of an ID close to the OD of your downtube, underneath of the U bolts, to spread the force out more. They /will/ slip though, if you catch much air or big potholes, unless you torque them down to where they start crushing the downtube. I ended up welding mine to the frame to make it stop slipping.
I'm not trying to be critical either! :) Just would hate to see all that work end up breaking and/or injury! I think this is going to be a really cool build (I've always had a soft spot for REs)!!!
I likely don't know what I am talking about, but if I were me, I think I'd redesign a little bit, or add more material, trying to get triangles pointing in different directions going on. I think that would resist flex a lot better than the squareness you've got going on there. Something heading straight from the downtube to the lower subframe, without the horizontal piece of threaded rod.
I'd be a shade scared that the tubing you are using is a little too small, too. Maaaaybe if it was round, but I think you've gotta go extra beefy with the square stuff...