1961 GMC 2500 Series Engine: 5.0L V6; Standard Rotation; Optional Non-Thermal Fan Clutch;. 1965 GMC 2500 Series Engine: 3.8L L6; Standard Rotation; Optional Non-Thermal Fan Clutch;. 1965 GMC 2500 Series Engine: 5.0L V6; Standard Rotation; Optional Non-Thermal Fan Clutch;.