How much H.P. would a set of headers give me?

I have a 72 Duster with a 400. Mild build, including: 452 heads shaved .060, Weiland dual plane intake, comp cam 224/230 duration 477/480 lift, 750 cfm holley, and i'm running stock type exhaust maninfolds. How much Horsepower should a gain if put some longtube fenderwell headers? Is it worth it? and how much trouble are they to install?