I have 18x10's with 295/35/18's on a stock tub Duster with a 1/2" spring offset. Section width on a 295/35/18 is 11.7 to 12". Looks like a 28x9" should have a section width of 10.8" to 11", which is pretty close to a 275 for a street tire.
Backspace on an A-body 8 3/4 with BBP axles should be 5.5 to 5.75" or so for a 10" wide rim. I have a 68-70 B body rear in my Duster, so I'm doing math to get that number from what I'm running. My numbers are coming from my A-body 8 3/4 with BBP axles in my Dart though so it should be pretty close.
