6.0psd777 - Just be careful, the blower motor slowing down does indicate voltage drop, which can be hard on electrical components in the rest of the truck.
Bad Night - I have the dual alt. option on my truck, and it's pretty nice having the redundency. I believe they are each only rated to 105 amp or so, but they do function independently. The PCM cuts out the lower one while the glow plugs are energized. The factory harness on the passenger side fenderwell should have a short pigtail lead that connects to a conector w/ a jumper wire for single alt. applications. The lower alternator field wires would then connect to the factory harness just behind the battery (instead of the jumper plug). There is also a fusable link on the charge wire that goes to the hot side of the starter relay. Sorry about the long winded explination, but the harness should be set up for it already.
I would imagine the bracket and idler pulley, and field wire harness would be a dealer item, unless you happen to come across one in the junk yard. The lower alternator is also not interchangeable with the upper one. Let me know if you need any more info.
Greg