A little late to the thread - been on vacation a month. 5000 miles in 3 weeks with wifey and canine on a GL1200/motorvation.
hhkiwi - your work looks great. I've almost completed my XR Sportie build after about a year. Struggled with the rear lower mount for some time. The XR frame has those same crossover members; but on the XR they are really light gauge. Maybe 16-18? I know that I distorted them when fitting my skid pan and using them as a mounting point. Hopefully they are thicker gauge on the earlier Sporties, or you may find your handiwork moving around unexpectedly. Following are links to some pics of my solution. Unfortunately (for me) on the XR HD eliminated the frame bosses for the mid-peg locations. I basically had some milled and then welded a pair of them onto the right side of the frame itself. Then spanned the bosses with a 3/8" think piece of steel that also incorporated the mid peg mounts.
Here are the bosses mocked up prior to welding:
Here is the bracket - the hack mount is lower left, and other appendages are for footpegs and rear mc. The top left also ties back to the frame for a 3-point mount.
This is the final that shows all the hack mounts:
And finally the hack frame connected:
Sportie frames are certainly a challenge to hack - but the result is worth it with gobs of low-end torgue...