Once the pieces were bent around the jig, I laid out my arc and began fitting and welding.
Unfortunately the jig didn't give repeat bends "exactly" the same. So I used a oxy-acetylene torch to gently heat, then bend, the pieces until they conformed exactly to my layout lines.
This is the first of four pairs of hoops. They'll then be stood up on site and joined by 3.5 foot long straight pieces of tubing to form a tunnel roughly 15' long.