Yup. If the perches are 44" apart center to center that's a B body rear end measurement. But if the 255's fit at all on 15x7's with 4.25" of backspace then it was shortened to A-body width, because that backspacing wouldn't work with those rims and tires with a B-body width rear axle.
If the tire and spring gap is the same though it probably isn't the perches being off center, it sounds more like the problem is the quarter/tub and not the axle not being centered to the frame. Body tolerances on these cars aren't great, the suspension points only had to be +/- a 1/4" from the factory. And if the quarter and wheel tub was replaced some point, anything is possible.