Underdrive crank pulleys will run all the accessories at a slower speed. This is good for the power steering fluid in autocross; it heats up and foams less. Otherwise, there's probably a minor improvement to rwhp, since accessory drag is less, but don't know how much.
I bought my '94 with an underdrive pulley on it, have never had a problem with charging, air conditioning, or steering assist. Worst case is idle, all lights on, A/C on, wipers on, brake lights on, radiator fan on - voltage drops to 12.5 -ish. Really not a problem.
The crank pulley is a 5-rib underdrive pulley for a 5.9L, and so the belt is also 5-rib (Viper is 6-rib.) Email me if you want belt PN or more info (it's at home.)