A few of us! This thread should help: https://www.v8buick.com/index.php?t...ttest-rear-18-tires-to-fit-72-skylark.366946/
Dave, rim size has nothing to do with back spacing. Back spacing depends on rim WIDTH. Back spacing requirements are the same for a 13" rim and a 20" rim with the same width. For a 70-72 Skylark/GS, 7" wide, BS 4 3/8" 8" wide, BS 4 1/2- 5" 10" wide, BS 5 1/2- 5 5/8" The wider you go, the chance increases that minor side to side differences will make a difference.