The "Whatever else it gets loaded with" sounds a bit ominous Walt.
You have a gross axle weight limiting how much weight you put in it you know.
Your back axle maxes out at 1550Kg.
How much more weight do you want it to carry?
Remember, anything right at the back, puts about 1.5 times it's own weight on the back axle.
A 50kg bag of spuds at the back puts 75 kg on the back axle, and takes 25kg off the front axle.
Madening isn't it.
But what it means is, if you make the back spring strong enough to carry "Whatever else you put in it", you end up with a nightmare.
Sorry if that's bad news, but you can only do so much.

Yes, you can make the rear spring as strong as you wish, to carry whatever you wish, but you'd end up with a driver's nightmare.

Mike