Your rig is about 15 yrs old. Not unsual for the springs to sag. I would stay with 450 spring, if you go bigger then the ride will be stiff.
Contact a Spring Shop. There should be one in your town. They will know better than any of us.
They might have fixes that work better.
Air bags are a cheap fix. I use air bags because it is a truck camper that is only on part time, so I can adjust the spring tension. I have been very happy with the air bags.