That center hole is called the center register. It REGISTERS the drum concentric with the axle rotation. Otherwise the drum can rotate around like a lobe making the rear shudder and really screwing with your braking.
IMO not advisabe to use anything but what fits properly.
There were machined rings to make up the space but I've seen mixed reviews on them.
are you trying to use 2.5" shoes on a 1.75" backing plate?
