The rear most mounting location, as you have discovered, utilizes a captured nut in the body. Fortunately, when I disassembled the front end of my '69 RS including removal of the sub frame, I didn't have any issues - everything came loose. Best I can offer is this pic which shows both lower bolt locations. Having the fender off still might be beneficial, but I'll defer to those who have dealt with this.
Good luck.
Richard