9:00 x 20 would have originally been put on 7" rims. Your 6" are really too narrow and wouldn't seat correctly against the rim edge. If they were 6:50 you could fudge enough but I would not do it on your rims. I agree that the 8:25x20 is as large as you should attempt with your rims.
From memory this is what I recall and I feel confident enough to suggest it.
5.5" = 7:00 tire
6.0" = 7.50
6.5" = 8:25
7.0" = 9:00
7.5" = 10:00
You can confirm this at any of the major repop tire companies websites.
