Underdrive or overdrive pulleys are useless if you're trying to gain power. The draw from things like the alternator or water pump is miniscule in the grand scheme of a daily driver. Race engines are a different story, but thats not what a Magnum in a truck is.
Differently sized pulleys have specific uses, largely, you will never reduce load on the engine. If you are drawing 800 Watts on a stock pulley, sticking a different one on there won't magically make your electrical draw less, you will still be using 800W of power, simply put it just draws that 800W at a different engine RPM.
The only use to change the pulleys, is to change the behavior of accessory components, such as to get better alternator output at idle, or to get better waterpump/fan speeds at low RPM for use in severe duty applications such as lots of city driving in southern climates or in snow plow applications.
