Convertible Belt...
Holy cow! Can you play music on that belt? The Convertible/Senior belts are WAY too small to get on those machines, even when used. I'd be very concerned about burning the bearings out in the motor or brushroll with that setup.
If you use the genuine Sanitaire/Eureka belts, they last a very long time. The generics will stretch out in absolutely no time flat. I've used both, the Generics may last 1-2 vacuuming sessions, but the genuine belts will last for months!