IIRR, Yes, you need the 3 bolt system because they are balanced differently. Additionally, I would not reccomed using any oem pulley's for other than a show vehicle. Reason: the pulley's are made from stamped steel, metal fatigue is setting in meaning the pulley can and will fail sending shrapnel (chunks of metal) through the hood, radiator and possibly engine accessories. These "faults" can only be detected by magnfluxing the components in most cases, however sometimes you can place a pulley on a lathe, spin at a slow rpm and check for visual deformities. A few uyears ago I was traveling back from a cruise night when 2 cars, both 1960's, T-Bird and Mustang, both experienced sudden pulley failures- onewhen through the hood the other through the radiator, destroying the battery, wiring and ac. I pulled my pulley's and found metal fatigue, replaced them with new aftermarket units (while I went with March for show purposes, less expensive units are available).
Member: Never trust a person over 40 who drives a Chevy club
Flatheads ain't so bad!
Certified backyard mechanic I & II