Welcome to the froums and the world of Vintage Airstream ownership. The information that you desire can again be accessed at the Airstream corporate site at its new address found below:
Airstream Empty Weights, Ball Heights, Etc.
I had been looking for this information for some time as it was displaced when the Airstream corporate site was reconfigured.
Based on my experience with my '78 Argosy Minuet, I suspect that the 500 pound bars should be nearly ideal for your Safari. The 400 pound "light-weight" bars might also be a possibility as well. I know the hitch weight on my Minuet runs between 410 and 425 pounds (20' with total weight of approximately 3,250 pounds).
Good luck with your search!