Kurt - My car isn't much help for the radius rod research. At some some point multi leaf springs were installed, with traction bars. Assuming the differential was factory installed, I can clearly see that a bracket was cut/ground off at the correct location. Attached are pics of differential stamp and castings. As for the rod itself being installed at the factory, we'll never know. I'll take a guess and say it was. Under the rear seat there is an area, directly rear of seat hold down bracket, that appears to have been stressed, cracked , sheet metal removed. Or maybe front spring mount stress?Someone laid a fiberglass mat over it, top and bottom. I might get a better idea if there was radius rod bracket when I get the fiberglass removed. It is what it is, as they say. She's found a good home and I'll enjoy caring for and driving it.
Woody