Let your alternators run a bit under fast idle.
There is probably nothing wrong with your alternators, they are just charging the batteries from when it wasn't before. The driver's side belt is somewhat a pain (at least for me) to tighten, so at normal idle you will have a red Gen 1 light and soon as you give it some throttle it will go out.
Remember alternators have different rpm's at which they actually start really charging the system, not all of them charge at idle, some don't charging until they hit 1800 rpms.
The resistor block on your firewall has nothing to do with your charging system, it is the step down resistor for your glow plugs, taking the 24 volts and stepping it down to 12 volts. Remember, not everything on the truck is 24 volts.
Try retightening the alternator if the red light bothers you, but it is a fairly common issue to have a red light dimly lit at idle, but nothing to really worry about.