Water Pump Pulley

Took the pulley off of my friend's car and it measures 5 5/8" diameter. I ran a piece of small rope inside of the pulley groove and it measure 15 3/8" overall length. I have no idea what this pulley came off of. It is currently on a 1971 Dart Swinger with a 1969 340 in it. I am trying to speed up the fan to help cool the car. Any help would be appreciated. The pulley on my car measure 7" across the back so I figure if I find a smaller diameter pulley that should speed up the fan.