8 3/4 swap question

I would bet the tube difference is the result of someone over tightening the u-bolts on the tube on that side - it would squish (that's a technical term) the tube and make the side bulge out in an oval shape. Measure tube width in both directions to confirm and then use a dolly and hammer to correct the out of round shape.

Get the proper u-bolt torque when you retighten them.