
I managed to get the brass inserts out by cutting through the screws (with the bar off) and then was able to get the inserts, with the rusted ends of the screws out of the plastic clamps. I even managed to get the rusted screw ends out of the brass inserts by clamping the inserts in a vise and using vice grips on the screw ends. I considered using "J B Weld" epoxy to glue the brass inserts back in the clamps, but wasn't completely comfortable with that.
I found "threaded inserts" at my local Home Depot, these are made for wooden furniture and have toothed outer edges and 1/4-20 threads on the insides. They were only a coupe of dollars for a pack of 4. First, I thought the teeth were threaded, but they are just little "teeth". I first tried tapping the holes in my clamps to a 7/16 machine thread. Then I found I had to drill all the way through the clamps because the "inner" ends (the ones towards the center of the car) of the holes were smaller. With the holes opened up to 3/8s I coated the inserts with J B Weld and pressed them in from the inner sides of the clamps so as to have the lip on the opposite side of the clamps from the pulling force when tightening the clamps. I also opted to grind flats on the lips of the inserts so they could be held with a wrench in the event the JB Weld didn't hold up. I finished up with a couple of 1/4 - 20 stainless steel machine screws with a Phillips head. I coated aall hardware with "Neverseize" before final assembly. All went back together and seems to work okay. Dennis