I obtained a 64 inch section of 2x2x1/4" square tubing. We notched out a section of the square spindle end to accommodate the seam on the inside of the square tubing. the spindle fits in nicely.
We attached the front hinge for the leaf spring on one side, installed the bushing in the leaf spring and temporarily mounted it on the trailer. I welded the hinge in place after checking that it was as true as I could get.
The rear hinged and its expansion joint were attached, lined up and clamped in place. Then the bolts removed and the hinge welded in place. The expansion joint is connected at about ~2 degrees over center to prevent it from moving in the wrong direction during a bounce.
The fully attached Leaf spring.
We also added gussets to the tonge for extra strength.