A 275/60/15 shouldn't have a 10-12" rim. Recommended rim width for a 275/60/15 is 7.5" to 9.5". Tread width on that tire is 8.9", section width is 11".
There should be ~12" from quarter lip to the stock wheel tub on a Dart, and with a 1/2" spring offset that distance is the limiting factor. So, a 10" rim would work, as thats 11" wide (outside to outside). But that's the max. But for a 275/60/15 any recommended rim width will work with a 1/2" spring offset.
Now, if you're talking about running 18" rims, a 275/35/18 is supposed to have a 9" to 11" wide rim. So, an 11" rim wouldn't work, but you could still run 275/35/18's on 18x9's or 18x10's, and that would work fine.