Wow, good price on those rims.
But yeah, 5.94" with 9" rims is going to be tight on the fenders with a 275/40/18. Might work with some negative camber and a little fender lip trimming, but it's going to be close. I'm looking at getting an 18x9" with about 6.2-6.4" of backspace for the 275's for my duster. Lots easier to run a spacer if you have too much backspace, pretty much nothing you can do but go with a smaller tire if you have too little.
On a Dart, 255 is about as wide as you can go without moving the springs. The 1/2" spring relocation should get you to a 275 without cutting tubs, but that would be tight. You'd need to have the backspacing just about perfect, and you'd still probably need to trim the quarter lips on a Dart to do that.
On a Duster you can get 275's on with the stock spring location, with the stock axle you need about 4.75" of backspace on a 9" rim to get a 275 to clear the springs and the quarters. I had my challengers 17x9's with 275/40/17's mocked up on my Duster with the stock springs and axle, they're a 5" backspace but I needed a 1/4" spacer to get them off the springs.
I'm hoping for a 285/40/18 on the back my Duster with 18x9.5's and 6.4" of backspace, but that's also going to be with a 68-70 B rear and the 1/2" spring offset.
