I am using 17x9.5 with 5.5 backspacing (275/40-17 tire). I had some bridgestone tires on it with a "rim gaurd" built in to them that would rub on the outer lip at times. The 9.5's are common on non-tubbed 67/68's but just be aware that not all tires are the same. The nitto's that I have now are not as wide and didn't have any issues.
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Chris
1968 Camaro SS
LS3/T56
Full of DSE and Holley parts