A general rule is the pin weight will be roughly 20% of the weight. That would be 2000 lbs of pin plus the weight of the hitch itself and any other goodies you put in the bed. Not to mention additional passengers, etc. Not sure about your 1999 GM ratings but there is a placard on your drivers side door that gives you your GVWR and maybe the tire ratings. You should check that figure first. 14 years ago I'm betting you'll be over the GVWR of your truck and maybe the RAWR.
Good Luck though!