Decent handling susp setup?

I think one source is called ESPO Spring, Hotchkiss is probably best. There are others. Contact Firm Feel and they will give you best advice for the level of performance vs cost. You can fit up to a 275-60 in an unmodified Duster rear wheel well with stock suspension and axle. Its very important to keep your mods a balanced package so that everything works together to achieve the desired results. The package I outlined above will result in a very good handling A body at minimum cost. Tires will be the limiting factor using 15s. Don't use stock alignment specs. They are for bias ply tires. Modern radial tires require different specs. I would ask the Mopar guys in your area for recommendations on an alignment shop that knows how to set up a older Mopar.