how about adding some airbags to the rear this should soften the ride look at low riddder magazines to find them cheap an air chuck valve like with the old air shocks should work fine and cheap. To prevent side sway have one fill line for each one. I know firestone has a terrible reputation now but they are still the industry standard for air bags and the only one I trust because they impregnated with stell like steel bellted radials.
of the the small air bags you can get a pair that will handle up to 15000 lbs apiece for under $ 300.00
the less load you expect the to handle the cheaper they are.
I use them in a tow truck no problems. smooth ride even when towing.
They even reduce body roll in cornering, this will help keep your plates on the kitchen tabel too
To get a car like ride out them ruduce the amont of leaves in your springs until you get the desired ride. If you can fabricate with steel and descent engineer you can make your mounts realitvely easily.
60 to 80 lbs of air pressure is usually about the most comfortable ride.
The only reason you should add more load carring capability than you need is due to the fact that when you reduce air pressure to soften the ride you lose some load caring ability as well.
about 15% more than your gwr is more than adequate.
I am building my bus with trailing arms the lower arms will be triagulated (to elimnated side sway ),air bags, sway bars and no springs period !will ride just like a car
even if you blow a bag they will not collasp completely.
If your wondering they are safe, big rigs use them everyday