This is a paragraph out of the ''67 Radius Rod Report. I would suggest you read the entire article. http://www.camaros.org/radiusrod.shtml
The one area of ambiguity is the L48/M35 model, which has sporadic radius rod application. There is no indication in the assembly manual that the traction bar was not applied to all SS-350 models, but other Chevrolet documentation indicate that it was not applied to the L48/M35 SS350/Powerglide combination. The CRG vehicle database provides insight in the actual usages. Of the approximately 100 L48/M35 cars built 012A or later for which we have rod-related data, 35% are reported with the round rod, and 5% with a rectangular (presumably) service retrofit. The remainder (60%) do not have a rod, but have one or more of the mounting brackets
There is no obvious pattern of application. Even cars with optional performance axle ratios do not have the rod.