Here's the thread on his wheels
GMachine Dart - New Wheel Package
( I have it bookmarked, his numbers are in the list of the references I keep to figure out backspaces and tire sizes :D )
I forget the name of them, but he runs 6.35" of backspace in the front and 5.95" of backspace in the rear with 1/2" offset springs.
For 275's on a Duster you wouldn't need the offset springs out back, but I would suggest a little less backspace even on 18x9's in the front. I run 275's on mine up front with 18x9's and 6.1" of effective backspace (13" rotors and a 3mm spacer). In the back it depends what rear axle you use. With an A-body 8 3/4 housing, BBP axles and the springs in the stock location you'd want about 5.25" of backspace.
