Make sure you are getting the proper timing/advance, air/fuel ratio. That 416 should be a beast; maybe take it to a dyno to find the right tune.
Aluminum heads are lighter, but will likely cost you power due to the heat loss, unless the heads have a smaller chamber/higher compression. You should probably consult a cam manufacturer and discuss what the best cam would be for your build, and your intended use. A 416 has a longer stroke and will generate excellent torque, but may not rev as quickly as a 340. For a street car, with the larger displacement and long stroke you may be happiest with a cam that produces massive torque.
I have iron heads on my 440 and a Comp cam with less than 230 degrees @.050, that car will light up the tires at will, and can chirp the tires going into every gear, but it is very streetable...(3.55 diff). If you have a race car, that is a whole different animal.