Jim Lee sells sanding mandrles to face off the back of the prop ,if you have a pitch gage ,set it up so that you can check pitch about half way out. if one blade reads say 5.5 pitch and the other reads 6.5 it is not square ,with a good course flat file thake the high corner off and check again, doing this until both blades read the same which in this case would be 6 in of pitch. then with a pair of calapers match the front to the back.