Choose your wheels carefully! They will be spinning fast and if not properly made and balanced could come apart with disastrous results.I would counsel against cart wheels unless they are sealed bearing units with poly tires . Also, you will find a 1/2HP grinder won't do the job (or even come close) to handling a 2X72 belt.(There are several other technical reasons not to convert a grinder,all have engineering words like side load,torque, and other technosphere words) Most people consider 1HP a bare minimum, 1.5-2HP is a better choice.Consider making a variable speed unit. The best way to go is with a DC unit of a VFD. Second best (and a LOT cheaper) is a three or four step pulley system (like a drill press head has).