On my 70 SS 454 El Camino, I not real sure but I think the bolts are 1/4" X 20. The plates are to protect the factory air bags on the " Superlifts " from road debris and rocks. All 70 El Caminos had factory air shocks or " Superlifts " more trivia F-41 suspension was not an option on any 70 El Camino. The option in 70 for automatic air shock control was G-67 and was called "Rear shock absorber level control " I just remembered the Aztec Bronze 66 from Canada. I had a funny look on my face in Buffalo N.Y. last when I found out where they were from. He said to get to Buffalo N.Y. they took a short-cut through Fargo N. Dakota. I thought they were from over the boarder Toronto or someplace. And they towed with a Silverado " V-6 " Hows That!!!!!!!! 1400 mile tow.....:hurray:
Bob