600:1 is huge. Are you sure you need that much torque? If you want to stay with belts, you definitely need to use the wider 18mm belts and pulleys.
To stay close to your original design (333:1 instead of 600:1) Vex has some great pulleys already broached for the 1/2 hex - smallest is 18T.
http://www.vexrobotics.com/vexpro/mo...tdpulleys.html Also they have light weight 60T
http://www.vexrobotics.com/vexpro/mo...rsapulley.html Most importantly, they have a calculator that is set up for these stock parts to help you pick the correct belt for you pulley pitch
http://www.wcproducts.net/how-to-belts/