Perhaps you have the very rare 9 foot stepside bed instead of the standard 8 foot long bed model. I believe there is a picture of one on this link about 1/3 of the way down the page:

http://www.6066gmcguy.org/GMCID1.htm

If this is the case I’m sure there will never be a reproduction as there would be extremely limited demand. I have seen aftermarket long bed steps, but they are listed as fitting 55-59 only. I’m not sure what would be different between 55-59 and 60-66, perhaps it could be modified to work. Here’s an example:

http://www.bobschevytrucks.com/1955-1966-bed-step-right-mid-size-bed-chevy-gmc-truck.html

Of course if you have a 9 foot bed it won’t help in any event.

1960 GMC 1/2-Ton Pickup Truck
In the Gallery
Updates in the DITY Gallery
Now cruising in the Passing Lane