There is a post below about an alternator conversion on a TO30.
He used a 39" x 22/32 belt and a wider pulley on the alternator. The end fit may differ with the design of the bracket, but it should be in the 39"-40" range.
Sometimes the generator pulley will fit the alternator, then you can use a wide belt, but you may not find one as wide as the original.
But, you can't run a thin belt that fits the standard alt pulley. It will bottom in the fan and crank pulleys and not drive properly.
If the gen pulley won't fit the alt, you can find a 5/8" wide pulley on Ebay or Amazon.