First- you will not fit 275/35/18's on a Dart with the stock spring location. You need the 1/2" spring hanger/shackle offset to make it work, and you'll need to trim the quarter lip as well. With the stock spring location the widest tire you'll fit is a 255, especially if you're lowered, and that will still probably need the quarter lip trim.
As far as backspacing goes, in the front with an 18x8 you'll want an offset between 18mm and 40mm, or about 5.25" to 6.2" of backspace. That will clear a 255/40/18, but 255's are usually only recommended for 8.5" to 9.5" rims. On an 18x8 it will be a little squeezed. A 245/40/18 would be a piece of cake. I have 18x9's with a 35mm offset and 275/35/18's on the front of my Duster with Dr Diff's 13" Cobra caliper set up. Same offset. I actually just added 3mm spacers to keep from just touching the frame at full lock.
In the back, you basically want a 0 offset rim with the stock spring location. But, if you want the rims you linked to fit with a 275/35/18 on an 18x10, you'll need that 1/2" spring offset. For that to work, I think you'd need about 6" of backspace. I believe that rear disk kit adds a little width to the rear track, so you might be able to get away with up to 6.25" of backspace. I wouldn't want less than 6" though, and more than 6.25" will need a spacer.
