Have a look at this Tech Tip
http://www.stovebolt.com/techtips/wheels/

If you think the 9.00x20 tires will fit the fenders and under the bed OK then the only other thing to check is the recommended rim width. The tire manufacturer will have a recommendation, or for a pretty good guess check the Accuride Wheel Catalog linked in the tech tip, they have a chart with recommended rim widths for different tire sizes.

Are you sure you have "split rims" and not just lock ring wheels? if Lock ring wheels there is not so much to worry about, check the tech tip.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-