Hi Dave,
Are they the castings from GLR? If so, I would guess that you are supposed to cut them in half acoss the shortest width, each 'lozenge' shape making two return bends, and then drill the cut surface for the elements?
I've always used Silverflo24 for soldering the return bends and have had no problems with the joints melting in use. I've now got some Silverflo20 which would probably be better and I will use that in future. My superheaters are always the stainless steel radiant type where they extend to the back the firebox so will get much hotter than the LBSC type that are only in the superheater flue. Once soldered, it takes a lot more heat to remelt the joint than needed to make the joint.
John