According to the Salesman's Book, the 7.50x20s were the base tire on the 6400. 8.25x20s front and 9.00x20s on rears was the max. I believe the tire size equates to the Gross Vehicle Weight, ie: capacity, but keep in mind the current tires are not likely the originals.
As far as parts on the larger trucks, the cab & engine stuff is all pretty easy, up to and including your local Napa store. Common replacement items such as wheel cylinders as well.
Some of the things related to the big trucks only, like the over-sized hood ornament, are not made by anyone, but those items are few.
Check the links page for good vendors - there's no shortage.
-Michael
