That bar/bracket is interesting. I have not seen it before. I suspect there was a design change. There was discussion a few years ago about an parking brake cable routed under the spring. Perhaps that '68 had the design too. I see that bracket in a Service manual view but not in the parts book. I know my cable is well above the spring and has never had mounting like that.

I only have run the stock 14" wheels and tires so I might suspect your wheel/tires.