Pics of pro-touring Duster with 20s?

Fine... Again, not a Duster but my Dart. 18 x 9" wheels all the way around with no more than a +5mm spacer. Dr Diff Stage 4 brake kit up front with a +32mm offset and a stock width A-body 8 3/4 rear end with Dr Diff's brake kit and a +22mm offset wheel. 255/40/18's on all corners. Some trimming will be required for the back as it rubs a tad on the fender. Other than that, no modifications.

In the back do you have any "extra" room to the springs? Would a little more backspace get you clear of the quarter?

I'm getting ready to look at wheels for my '71 EL5 GT. I'd like to go with 255's all the way around, but I think 255's are going to be tight out back unless I go with a 1/2" offset kit.