I'm driving it around with the 275's now and they rub a bit on bigger bumps. It seems fine with the 255's but as you say, I might not get them "tucked in".
I have an A body 8.75, 489 case with a 4:10 Sure Grip but no axels. I have 3.55 gears now which are fun...you know how it goes
The rear came with the car soon as I looked at it I knew something was wrong. Not to mention the springs were shot, no bushings, missing binders. I got a good deal on the springs from a local guy who was cleaning house.