400 is too much...
At my shop we will take leaf springs that are broken or the center bolt has come out and put them into inventory.
When someone needs a lift, we would measure his or her original springs from the end to the center and from the center bolt to the other end. We would then look for a spring that would fit below the main and above the top leaf. If we could, great, if not, we would cut down one we found that was longer.
Being a van you will have a 9/16 center bolt, cars have 3/8-inch bolts. For us to do the job we would "TRY" to save the U-Bolts. Pretty good at it but if not, then we would replace them.
Basically, for 2 long leafs on each side plus the labor we would charge on average about $150.00 plus any U-bolts and the new center bolt. U-bolts should go $11.00 to $17.00 each and the center bolt less than $5.00
Any local shop should be able to help you. If you have no salvage yard right near you, then you may want to make the run around to find some long leafs to help with the total cost from the local shop doing the work.
If no one around you can or will help you I can.
Simply e-mail me the measurements as I described above. I'll have my shop cut them and ship them to you....
Let me know...