1967 cam/firebird base models were equipt with mono leaf srings, which I know your aware of unlike the multi-leaf springs of the model years there-after the spring saddles on the axle were deeper. When up-dated a '67 the saddle should be replace or install a 68/69 axle['70 will not fit as the saddles ar wider] Since that has not been done the centre bolt is all thats holding the axle to the spring and the saddle is only contacting at the first two leafs. The dual "U" bolts is good insurance, but it looks as though your just about out of thread! Maybe some correct Bolt migt secur them better.