largest rear tire on stock wheelwell 69 dart

Yup, it's pretty common for the rear axle not to be perfectly centered between the wheel wells. They're not usually off by more than a 1/4" or so, but even that's more than enough to make tire clearance on one side a problem.

And even the wheel wells themselves can be off by a little, on my '71 GT the axle is centered but there's an extra 1/8" in the width of the drivers wheel well compared to the passenger side, it's just how it's assembled with the quarter, wheel tub, frame rails etc. Not a big deal if you're ok with trimming the quarter lip a little to even things up, but if you don't want to trim anything at all you have to be careful.