it may not be year ,model correct but the early 80s diplomat/5th avenues use a double pivot linkage system that i really like you could dig up at a bone yard there all 318/727-904 cars im sure it would fit your setup

67 charger fastback 318 stock 67 belvedere II 4dr soft top built 318 68 newport 2dr hard top 318 84 b150 440/727 84 d150 short wheel step side 4x4 lifted 87 omnitruck (you just have to see it ) new addition 63 dart wagon !!!