I run 18x9's with 275's on a +35mm offset rim on the front of my Duster, but my brakes add 5mm to the track so effectively it's more like a +30 offset if you're using the '73+ disk spindles as a reference.
Whether it will work on your car though I don't know, because you still haven't told us what brake kit you're running or how wide the rear axle is. You said the car is mini-tubbed, but you didn't confirm the springs were moved 3" inboard, and you haven't measured to see if the axle is the stock width. With an A-body 8 3/4 axle width if the springs are in the stock location still a 6" backspace won't clear. If the springs are moved 3" inboard they probably will clear, but not if the axle has been narrowed to match.
I don't have a crystal ball, so, without measurements it's really hard to say.
