theres no way you'll get that 275 under there unless you totally inboard the springs from what i know...the demon/ duster and cuda wheel openings are much more forgiving than the dart/scamp style ones, so you are limited
And hence my suggestion (above) to measure, measure, measure before doing anything else. You are limited with a Dart-style wheelwell but I could have sworn I saw someone get a 275 in there wo/inboarding and just using the perfect offset on an 8" rim w/the offset kit. I could be wrong and know that many use a 255 in the rear. It doesn't hurt measuring and figuring it out.