Yup, I totally agree. If that was an A-body 8 3/4 with 15x8’s that have 4.5”
of backspace then 255’s should fit just fine. That’s not an A-body 8 3/4. I would bet that’s a B body rear axle in there.
That brings up another issue though. The rims may not have enough backspace for your rear axle, but they are the right rims for the car. And to get more backspace on a set of 15x8’s you will likely need custom backspace wheels.
The shackles have to go, those extended shackles screw up the suspension geometry and don’t work all that well anyway. Fun fact about the leaf springs though, they were designed to be zero arch springs. So the fact that they’re almost flat at ride height means they’re not far from where they normally are anyway. Guys that run SS springs are used to seeing a heavy arch on the springs, but that’s not stock. The previous owners no doubt tried to raise the car to make up for the fact that the wheels stick out too far and hit the body. With the right wheels, you wouldn’t need to raise the car.
