Doug's exhaust help '66 Dart

Hi everyone. I'm pretty new to the site, but I have been doing some research for headers for my 1966 dodge dart 270, and think that the Doug's D450's will work best for me.
First, the kind of motor I built was out of the Engine Master magazine.
It is a 318 with 360 heads with the keith black pistons, etc. (the engine master's guys got 402 hp from this combo)
The only problem now is that unfortunately since I couldn't afford the Doug's headers at the time, I'm running the stock 273 exhaust manifolds.
So I was wondering how much of an increase in horsepower I could come to expect by using the Doug's headers? and when installing them, does anyone know if I will have to take apart my steering linkage or torsion bars, or is it possible to put them in without removing a bunch of stuff?
Thanks for any help...Kory