65 barracuda need help with rim backspacing

Are you moving your springs in to the frame rail or just using the offset hangers? I don't think there's an advantage to moving them in to the frame rails unless you plan to enlarge the wheel tubs, which will require alterations to the area under the rear window in an early Barracuda.

I'm kinda' in the same situation as you are. I have a set of 15 x 8.5" wheels with 5" backspacing (6mm positive offset). If I use these wheels with a stock A body 8 3/4" rear & offset spring hangers, I think the wheel will be nearly centered in the wheel tub. I think I might need to narrow my rear slightly (maybe an inch or less, I haven't made my final measurements). I'm hoping I can do this & still use the stock axles by switching to "green" style bearings & removing the thrust block to let the splines go a bit farther into the diff. I'd really like to fit a 275 tire in back, but haven't decided for sure yet.

I think a tire 28" tall will fit but I might need to move the rear axle back a bit to center the tire front to back in the wheelwell. This should be simple when using the Mopar spring perches since they have an extra set of holes in them. I'll need a longer driveshaft but I need to have a new one made anyway.