Max backspace on 9 inch wide by 20 rims?

I found a set of 20's w/ a 5 inch backspace. They are 9 inches wide. Do I need a 1/2 inch or inch spacer, and can I use the ones that are like washers using same studs, or do I nedd to get adapters 5 by 5 to 5 by 5? If I do will I need to grind the studs? Thanks Guys!