50 amps or 60 amps

Hey guys, i think my alt. has bit the dust so i'm replacing it. I curently have a single groove pulley on it now, and the belts on the groove closest to the motor, and the second one isn't used. On alt is a double groove running at 60 amps, and the the other is a single groove running at 50 amps which way should i go?? It's a 71 duster 318 Daily Driver.

Thanks
Adam