i don't think offset really matters when the wheel is only 3.5". but it should be -13mm with a 1.75" b/s. it really depends on what you're willing to do with the rear. there are people on here running 315 m/t drag radials. you definitely have to roll the lips and massage the inner fenders though. i'm factory 4 lug and i have 275's on a 10" rim on the back of my car with no modification needed other than removing the quads which you said you already don't have. a lot of it has to do with your axle setup as well. are you factory 4 lug?
i don't think offset really matters when the wheel is only 3.5". but it should be -13mm with a 1.75" b/s. it really depends on what you're willing to do with the rear. there are people on here running 315 m/t drag radials. you definitely have to roll the lips and massage the inner fenders though. i'm factory 4 lug and i have 275's on a 10" rim on the back of my car with no modification needed other than removing the quads which you said you already don't have. a lot of it has to do with your axle setup as well. are you factory 4 lug?
well i'm running 17x10's with a +25mm offset and a 6.45 b/s....it'll be a little different running a 15x10 with a 275, you have to consider tire bulge, although i honestly dont think a 275 will bulge very much, if at all, on a 10" rim. you should be fine with a 15x10 +25mm offset 6.5" b/s though. i highly doubt any inner fender massaging will be required, you might have to roll the lips though. svt32vdohc is pretty knowledgeable on this subject, as well as many others here, they should chime in soon enough. on an old GT i used to have, i had 275/60/15's on 15x8's on the back (stupid truck tires my bro put on one day when i let him borrow it) and they rubbed the quads a little going around turns. other than that, no issues.