1983 E350 Class C Motorhome Headers

I was wondering if anyone has installed headers on 460 in a Van chassis? I have a broken bolt in my head and am burning my plug wires. I was planing on pulling the motor to fix this problem. I found a brand new set of headers with installation kit on ebay and got them today. How much more power did you gain and how about gas milage.

I guess that would be a NO?