If you move that wall 1" you'll also have to do tilework. I 2nd the custom vanity suggestion.
When I replaced my kitchen cabinets I did not realize the walls weren't quite square, so measurements in 1 location did not match the width everywhere. I could not get the cabinets at the back of the "U" to fit between the walls. Lots of 4-letter words later, I carefully cutout a section of 5/8" drywall on each side and replaced it with (I think) 1/2" drywall, making sure all the seams would be completely hidden. A little pushing and prying (and lots of shimming), and those things are in there SNUG!
Edit: I have to wonder what the builder used, and how 3 base cabinets add up to 71".