It's the width of the tires that makes you need the proper offset/backspacing. On a 20x9 wheel you need at least a 0 offset (5" backspacing). A -12 offset will work too, just stick out a bit further (0.5").
Here's a link to my thread installing 35x12.5 Toyo MT's on 20x9 -12 offset wheels. Yours would stick out just as far (if you go with a -12 offset), they just won't be as tall.