Alternator

To keep the Magnum serpentine setup, use the Magnum timing cover (no mechanical fuel pump provision), Magnum alternator and bracket, and wire the Magnum alternator just like an old school electronic-regulated alternator. Your old ignition wire goes to one of the small terminals, the other small terminal goes to the controlling wire of the solid state electronic voltage regulator.
If the car has the old mechanical regulator, upgrage it to electronic for best results. I have not tried using the mechanical voltage regulator setup on a Magnum alternator, but in theory you could wire it that way, in which one small terminal is grounded and the other comes from the mechanical regulator.
As always, refer to appropriate wiring diagrams for your car, as I am going by memory here.