Rubbing issue with 20x8 255/35/20 rears

[OZ Duster]

Hi all;

After getting my new acquisition, 69 Camaro, on the road I have noted that the tyre is rubbing on the inner wheel well when under side loading, specifically the inside rear wheel.

The rear set up consists of what I believe is a standard width diff, Wilwood rear disk brakes, AR Torq thrust 20x8 (zero offset) 4.5" back space with 255/35/20. The track measures up at approximately (tape measure) 61 inches. not sure if the disk brake change on the rear has changed the track width significantly?

There is no visible sign of rubbing on the tyre however, there is some "polishing"of the inner wheel well. It sounds more of a short squeak than a heavy impact or lock up.

A search on here has uncovered some standard cars running some serious
width rubber under the rear of 69's. Beside the fact that it rides like puss, I seriously want to reduce the wheel diameter down to at least 18" all round possibly 17",

Until I can decide what wheel I want, would the short term option be reducing the rear tyre to a 235/40/20, not ideal but should stop the rubbing?

Any suggestions?

Cheers