Have you read this manual, your gear box will very similar.
https://www.flickr.com/photos/advance-design_parts_co/sets/72157632210242370/

The packing and retainer can be replaced with a simple seal from a newer GM manual steering box. This is the seal, you may find it local as well as it was used for a lot of cars and trucks. http://www.ebay.com/itm/SKF-11055-Steering-Gear-Seal-/170840391774

The original seal was a felt like material with a steel cup to hold it in place, see manual above.