That was going to be my point. If it is for towing... No reason to have a horrible over sprung suspension all the time. I don't think you realize how stiff those springs are. To want more is nuts. You need to buy a 1 ton truck if you are carrying that kind of weight in the back.
I have the 897's which are a shorter version of the 899's and I agree with the rest on using airbags for towing. 897's are borderline too stiff for even an additional 600+ lbs.