The seals are easy to get, GM used it for many, many years. Just ask for manual steering box seal for a 60's or 70's GM gear box.
To get the old one out, drill a small hole in it and run a small screw in it to use as a pulling point. They will come right out.
On the other hand, pulling the whole gear box out for a overhaul is the best way. A good cleaning, adjustment, and new grease will work wonders for the old truck. Its a fun project thats not to tough, but makes a world of difference.
Joe
