I did the same project on my 76 super sport a while back. Fabricated my own seat rail tho, as I felt the DCc setup was a little pricey at $80 or whatever. Basically I cut the tabs off the rear of the frame just past the rear gusset on underside of the rear shock mounts. I got some slugs made for the inside of the joint, rosette welded them into place. I then beveled (chamfered is the proper term I think) the ends of my home made piece to give a nice groove to lay the weld down into. Slipped the rear section over the slug and welded her up. The pic shows the frame section finished and the seat pan I fabbed slapped on top. Second pic is the seat upholstered.
One thing I learned, not totally the hard way- make sure the end of your loop extends beyond the center of the rear tire, otherwise you'll screw up your clearance and risk bottoming out all the time. My rear tire ate that license plate first time I threw a lady on the back. Sorry officer...