Easily the 275/60s like you're wanting, probably up to a 315.
But I think you can fit the 275/60s with just a spring offset and picking the perfect rim size and offset. There has to be someone here with a Dart or Scamp that's done this tire size without a mini tub.
I have a '70 swinger offset spring hangers, rolled the outer lip a little and fit 275 / 60 / 15 on a 15 x 8.5 rim with 5" backspace. I can't fit my finger between fender or leaf., but it fits