Appropriate Wheel Offset for 18" Wheels?

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. I test fit the 275's from my Challenger on my '71 GT, with a 275/40/17 and a 17x9" rim and 0 offset (5" backspace). I was physically able to get them bolted onto the car, but they were actually touching the quarter and the spring. Just won't work without a 1/2" offset and 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, so I'm at 32mm offset on my 18x9's. I did still need to have the hub bore on the rims machined to fit over Dr. Diff's hubs. They're smaller than the stock hubs, but they still aren't small enough for most Ford or aftermarket rims.

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.