Hemis used a thicker harmonic balancer an that pushed the assembly out further. So the double alternator pulley made the lineup work. Hemi pulley are hemi specific other than early water pump pulley being a Normal BB pulley, and the alternator pulley actually was the same as the early 60 AC car pulley. Crank pulley were specific and in 68/9 up the water pump pulley and the Federal PS pump pulley became hemi specific too. Plus they were altering drive ratios since the hemi would be expected to turn at higher RPM under expected racing conditions.