re-arch & re-temper
Find a spring shop that specializes in re-arching & re-tempering springs. They could safely rebuild your leaf springs to match your present vehicle requirements. Would probably require weighing the vehicle. I have no idea what that would cost these days, my brother is a 4X4 guy and has talked about a custom set up for his land cruiser. You could convert to air bag but your handing will suffer more than than with a re-arch.