A very difficult and tricky thing to explain.
The problem of hard suspension of an sj 410 can be solved but it does have some negatives.
Like the stock leafs are 3 in front and 5 at rear (less the jumper leaf). So if you just make them 3 in front and 4 genuine springs in rear, the ride will slightly improve but the problem you will face is that the veh will get lowered once loaded (means 4 to 5 ppl) and the rubber pieces on top of the tubes will hit the frame.
Same is the case with the coil springs. The ride wil improve but once loaded it will be a problem.
And this is the problem with all light weight vehicles that more leafs make them hard and both less leafs and coils make the ride soft but less load bearable.
What i used to do is to keep stock leafs, means 3+5 and would use 15" rims and bigger tires so that on events of IJC i can reduce the tire pressure and get a comfortable ride. The stock 6X16 general tires can also work bot if u keep less than 18/ 20 pounds of air pressure.
Going for coil spring is just as you reduce the no of leaf springs. And just to explain, if you keep only 2 leafs at front and 2 at rear, the drive will be like a crown, but than weight lifting will be a problem. I hope something is clear?
Just to give you an example, a person went to a skin specialist to show him pimples on his face. The dr asked, what do you eat? And the reply started with 5 eggs in the braekfast followed by a long list of hot meals. The dr asked why do u eat so much of protein and the person replied that he does body building also. The dr than commented that, " either you can become Shah Rukh Khan or Arnold".