Hi Flyboy,,,,
I have an 85 Sierra 1500, it has the factory step bumper, the tow rating on the bumper hitch is 400lb tongue weight and 4000lb tow weight. This however does not mean that you can safely tow 4000lbs with only the ball on the bumper, I have pulled a 2 horse trailer which when loaded was probably right at the limit and I know it would have been unsafe at hiway speeds due to trailer-sway, and the front end of my truck felt very light. YMMV