You could make it work. The spring in the tracking wheel is pretty light. Fine for a 42" woodworking sander, but for a 72" where you'll want a lot of tension you'll want a stronger spring to pull the belt taut.
Sunray wheels will make a polyurethane covered wheel that is keyed, 70 duro, and slightly radiused. A 4" drive wheel will be under $100, but I think they have a $100 minimum order.