I am confused, it happens a lot:elkgrin:, why use a hiem joint in this application, the motion is only in one axis?
This seems like a better option
That would be my vote also, but in the OP's design, the swing would be a double shear.
With the spindle, he would have to stack the square tubes. (very doable)
I agree, a bushing would work just as well the hiems.
Either way, I would go with 3/16 or 1/4" for the plates (blue color)
I've use 1/8" before, and it didn't take long before the holes started 'egging' out.
Also, keeping the tire offset a bit to the 'swing' side, will help alleviate most of the rattle and clunks from the hitch, as the weight will be off center.