If you are using Reese Dual Cam Sway Control with your setup, there is another reason to go with the spring bars that are rated just a little higher than your maximum hitch weight - - at least according to the Reese Technician who assisted with the revisions to my setup. It seems that the engineering of the lever action depends to some degree on the spring bars being rather evenly matched to the tongue weight of the trailer involved.
My '64 Overlander has a hitch weight of 725 pounds when fully loaded for an extended vacation, and I have been utilizing the 750 pound bars that the Reese Technician sold me to replace my under-sized 500 pound bars that a Brand X dealer had sold me based upon the published hitch weight of the unloaded trailer. In nearly eight years, I have found the setup to be extremely stable with no tendency toward sway.
I have just finished the setup to use Reese Dual Cam with my '78 Argosy Minuet/'75 Cadillac Edlorado Convertible combination. Right now, I am estimating that the hitch weight when fully loaded will be 450 pounds to a max of 475 pounds. The spring bars that came with the trailer are 500 pound bars so it looks like it will be a good match.
Good luck with the decision regarding selection of spring bars.
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)