You measure circumference actually. The difference should be within .25 inches. It'll be out of spec for sure.
Shaving might work, but the current tires are no longer made.
Unless the tires are different sizes to begin with, tread depth difference is a perfectly acceptable measurement. Some use 2/32" left to right and 3/32" front to back. Others like to use 2/32" difference in any direction.
"Another Brick In The Wall", PF 1979
"Happiness Is A Warm Gun", Beatles 1968
"The Rubber Banned Man",Spinners 1976
2011 SS Camaro