Tire Size Question on 68 Dart

I have a 72 Dart Swinger with offset shackles,8.5 rim with 4 7/8" backspace and 275/60/15s under it.
The lips of the guards are folded up and my diff is 1/2" wider overall than stock.
It fits tight but nice.

I have had my slicks under there which are on same size rim with 28x10.5x15s (equivalent to abt a 305)
They fit but are too tight.Any suspension movement would see them rub.

Only 2 pics I have showing you the street rubber below.





Oh...I also relocated the front spring eye 1/2" rearwards so the 28" tall tyre clears the the front lower corner of the guard.