You have 3 options really.
1) You can ignore stretching the frame out 6mm and run 2 snail cams regardless, which can't be good on the frame really.
2) Do what Adam said and use one snail cam on the drive side which will work fine. As he said, Stan was using it for a while like that.
3) You can get them machined out 3/4 of the thickness so they slip over the end of the axle if you are really that bothered about frame stretch.
If you are unsure of measurements to go by or don't have anywhere to do this for you, contact 'Bigman' on here and he can sort this for you.
EDIT: Yer also Adams last point he made. Drill the holes bigger to fit all the way over the axle.