From the sounds of it, all you may really need is an appropriate dual rate spring weight for the load you are carrying. I would recommend calling Andy at RacerTech and tell him what you have and what you need it to do. At a minimum he should be able to recommend the correct spring rate to support the load you are wanting to carry. I know he carries King and Elkas and some a very custom shocks (like up to $500 per shock) or just springs and spacers ($100 each approx.) as needed. Springs - Shop by Part
I am sure they do not have anything in stock that fits you needs but can t least tell you what you need and may be able to get it for you.