As strange as it may seem, the spool valve in the steering box is actually a two piece design, held together by an internal pin. It sounds like that pin has sheered or is loose, causing the internal shaft to "spin" inside the outer housing. Best to go to a wrecking yard and pick up another steering box.
The spool valve is what the intermediate steering shaft attaches to at the top of the box. It can be changed without removing the steering box from the chassis [if you can find one], but great care must be used to keep the lower shaft in place, along with the worm gear and recipicating balls in place on the shaft. Best to get another box and just replace it.
If the belt slips, try using a 10mm wide belt, or a 7/16 belt instead of the 3/8 belt. It will sit higher in the pulley groove and have a better grip of the pulley