ahhhhh.....
It appears you are planning on transmitting the torque for driving the wheel thru the threaded rod, being drivin by the hex shaped coupling nut on one side and driving the wheel with the hexed shape coupling nut on the other end.
Frankly, it doesn't look like a very durable design....I would try to come up with a different design, but then I have an old South Bend lathe sitting in my garage.
If you can put metal spacers between the bearings and between the bearings and coupling nuts, so that you can actually tighten the coupling nuts against each other solidly, then it might work better. Having a long nut like that will probably let loctite work well enough to keep it from turning, although it may work so well that you won't be able to diassemble it for service. A pin thru the nut and rod might work, but would weaken the threaded rod somewhat.
Also the frame tubing could use a metal spacer over the bolt that goes in the center, to keep from crushing the frame when you tighten the bolts as needed to keep the custom bearing blocks from moving around.
Sure would be nice to find some hex shaped tubing, eh?