Did that rear frame horn replacement myself many years ago; almost the first repair I did on my own SS. Went with the belt and suspenders approach by fabbing up two pairs of fish plates; one for the inside of the rail and the other for the outside. Drilled the frame stump and plug welded the inners into position and then hammered them into shape. Slipped the new horn onto them and trimmed it to both fit for location and for edge to edge with the stump. Used an inner rear bumper channel as a locating fixture. Drilled the replacement for holes for plug welds to pick up the inner fish plates. Welded it all together. Ground the welds down to flat, drilled the outer fish plates and then did a combination of plug and seam welds to secure them; hadda pound on them a little for shape as well. Lots of fun while sitting there on the back yard driveway in the sun and rain; no shop back then; just a little sheet metal shed for storage. Crowded, but still a lot more simple than what replaced it.
Nick