I have used a spacer made by Arrowcraft and they were really nice. The inside bud wheel goes on the hub first so it is centered on the hub ring. Than the adapter goes on and is bolted to the original whell studs. Finally the outer bud wheel is bolted onto the adapter; it has a centering ring machined into it for the bud wheel.
In my opinion weight capacity must go down and the extra leverage created by the adapters has got to tax the wheel bearings, but if extra floatation is needed with wider tires than they do there job.
The Arrowcraft stuff is really nice and very strong and well made , but it is not for all applications. Good Luck .