well a sraight 8 pull around 150 amps .. and a 12 pill pulls 215 well thats what fat boys site says so if this alt going to be only the amp ?
An 8 pill might need 140-160 amps but you are forgetting the driver amp. 2x2290, add 20-30 amps. 40-60 amps for the truck and you are most likely at 240 amps. 12 pill plus driver and trucks power draw......320 amps.