Put everything in a 10-quart stockpot, cover and bring to a full rolling boil. Once boiling add the salt, reduce the heat and simmer uncovered for 3 hours until the stock has reduced, stirring occasionally.
Strain the liquid through a cheesecloth-lined sieve into either another pot or some other large (6-quart or so) heat-safe vessel and let it cool completely to room temperature before covering with plastic wrap. Place in the fridge and allow it to get cold so you can skim off the access fat easier or, once its cold, strain it again through a fine-mesh sieve lined with cheese cloth to help catch the fat and any stray solids there may still be (and for the latter you should probably do this).
This will keep for a few days in your fridge and up to 3 – 5 months in the freezer.