I've made spacers out of flat aluminum plates, anywhere from 1/4" to 1/16" thick, and used whatever stackup was necessary to make the belts run true. Washers won't support the pulley well enough to prevent wobble.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!