Welcome to the forum. This link will take you to the Service Manual section of the Technical Library where you will find a separate section that contains all of the Service Manuals for the 130/180 Case tractors including one for STEERING.
In the Parts Manual section, you will find the original parts manual for your tractor and in the Operator's Manuals section, we have the original manual that covers the 130 and 180.
You have not stated what is wrong with your steering box but all of the parts are available to rebuild it. Ross boxes were used in quite a few GT's in the 60's.