65 Barracuda Factory Fan?!

There is room for a stock clutch setup. The clutch takes the place of the long spacer that a non-A/C system uses. As for the two different designs of "clutches", the fluid drive unit is just a silicone based unit that will not completely lock up. The thermal drive unit has the bi-metal spring that attaches to an actual clutch inside that will lock to the water pump, spin the fan faster, and give more potential cooling effect under certain conditions. It also has one more seal that can potentially leak.