Several members here have modified an OEM pulley from a 74mm to 70mm, maybe (?) a 69mm. I believe anymore, and you're going to cut yourself thin on stock material for proper groove depth, and maintain strength.

I'm not sure if stock pulleys are thin cased hardened or not. If I ever remove mine for a bearing replacement, or pulley swap, I'm going to check with our Rockwell hardness tester just for kicks.