You're in a similar boat I was in. My 66 Dart had the same coupler you're deadling with. Luckily, mine had the two rectangular springs and they were in good shape - as were the shoes and coupler body. I simply replaced the seal and installed a set screw to act as the small safety pin on later couplers.
Maybe try placing a wanted add for those two springs. Are the two shoes and body in decent shape?