Well if they're 2" lift springs that would certainly explain the arch and why the car sits so high in the back. The best way to deal with that would be to have them de-arched
Which B-body rear axle do you have?
Cop wheels are 15x7 with a 4.25" backspace. With any B-body rear that still won't be enough to clear 255's on those rims with you lower the car, let alone 275's.
I have a 68-70 B-body rear axle in my Duster, and even at stock ride height the most that would fit on cop wheels was a 225/60/15, and they were very close to rubbing. The 68-70 B rear is 60 1/8" wide, even if you have the shorter 65-67 rear that's still 59.5" wide, so you'd only gain 5/16" in clearance per side, which probably wouldn't even get you to a 235 unless the quarter lip is well above the tire, like it is now. To clear 275's on a stock height or slightly lowered car you'd need a lot more backspace.
