At least the junkyards around here sell wheels at about 50 dollars on average each. You might just get some junkers without tires you feel fine about running without tires. I bet you could get cheap Chevy six lug steelies for a good price. Just make sure they have the large enough hubs.
So... Not saying wheel dollies are the best but i have set the axles on 6x6 boards that were on wheel dollies so i wouldn't put weight on the brake backing plates . That was lower. My wheel dollies have brakes on them. You could go with 4x4 boards just make sure you can get the jack out and back...