Just to confuse matters a little bit, I just found a different chart in a 1945 catalog that says a 6.50 tire is recommended for a 5.00" (7") rim and permissible for a 4.33" (6"). That would let you put the 6 x 20s on your two narrow fronts and mount 6.50s on your present rears. (You sure had way too much tire on your rears!)
Looking at the Universal Tire on-line catalog the 6.00 and 6.50 sizes are similar outside diameter, 32.4" and 32.6" respectively. Here's the link:
http://www.universaltire.com/cart.php?target=category&category_id=343&pageID=1 Last thought, if you can pick up the other wheels cheap enough it'd be worth doing to my mind. Until you get your current ones unmounted and sand blasted you won't know if there are cracks, rust issues, or rivet damage. Especially with that much tire on the rear I'd be looking for overload damage. Having spares is sometimes a life saver. Stu
