Not a stupid question at all. Actually can be complicated.
Yes, moving the springs in would allow for a 5.5" backspace. More actually, I'd want about 5.75". Peter runs his 18x9's with a 5.95" backspace and a 1/2" offset in the back. But that's where it starts to get more complicated. I haven't seen his car, but I would guess that he might need a little less backspace than that to run 275's and keep them off the springs because they'll hang out over the rims more. Based on the math I'd want about 5.75" on 9" wide rims for 275's. That would give the same clearance to the springs that I have on the 295's on my Duster (not much!). But that's working off the math because I'm running a B body rear axle in my Duster and don't have anything that exciting on my Dart yet.
As for how much fits, it depends on the car. On a Dart a 275 is the pretty much the max with a 1/2" offset. On a Duster, well, it depends on how much you want to cut. I run 295's on my Duster with a 1/2" spring offset, but I also cut almost a full 1/2" off the quarter lips. A 285 would fit most Dusters/Demons/Dart Sports with stock quarter lips with a 1/2" offset. And that should be on the conservative side, my car sits low. With a higher ride height you'd have more clearance because the widest part of the tires would stay below the quarter lips, unlike on my car where you have to figure the clearance from the full section width.
