Will this mandrel be ok with your Kingsly north 8" diamond grinding wheel and your 8" diamond resin wheels for cutting and polishing opals. Also will i be able to attach a polishing padon the end?

You could, but you're going to have to mount in a way that elevates it on your workspace since as it is naturally only leaves about 1" of space below the arbor. You're also going to want to make sure it's balanced so if you run one metal bonded wheel on one side, you should run another on the other side as well. The length of the V belt (1/2" A-groove) will be determined by where you mount the motor and what size motor pulley you use. The end is not tapped so you'd have to get something that would go over the arbor rather than into it.