elongating holes on shock plates

I just got done sandblasting my shock plates tonight and stated to work on them. I am swapping form the 7 1/4 to the 8 3/4? How does everyone on here elongate the holes. I tried using my air tool with a grinding stone? but i borke one and wore through two others and still havent got far enough on even one hoes. There has to be a better bit or differant way. Any ideas? thanks
mark