Following up for historic purposes - in case someone else is searching for info..
The rear spring perch reinforcement is documented on 1954 Kasier-Willys Service Bulletin, #246. The kit contains simple reinforcement plates that are welded to either side of the 'frame rail,' as well as a new shackle that is 0.50 inch wider on top. I was not able to find anything for the shackle part number - hoping that it may have been adapted for other Willys models over the years.
Without the unique shackle to accommodate wider frame rails, I had to adapt my repair to fit within the existing shackle width. I opted to modify 1.125 inch washers that had an OD just inside the existing recess on the outer frame rail. I welded those to the original bushings that were tack welded in the frame, and provide a larger area, outside the damaged frame rail to weld to. The damage to sheet metal was only one-sided on my car, so this worked for me. YMMV
Photos attached