Difference in Fan Clutch w/Rear Gear Ratio ??

I was looking at an aftermarket fan clutch for my '70 340 Duster and noticed that they showed one fan clutch for a 340 car w/271/276 rear end gears and another clutch for 355/391 gears.What would be the difference?Both are showing to be a 6" diameter clutch.Why wouldn't they interchange?Just courious

The factory parts book breaks them down the same way, one clutch is thermal, the other isn't.

Good question.

B U M P.

So a thermal fan clutch has a raised center and a non-thermal clutch would be flat?

In the aftermarket,most thermal fan clutches have a bi-metalic spring on the radiator side that changes the orifice that the fluid in the clutch flows through, increasing or decreasing drag. Non thermal rely on simple fluid friction.

Any idea if they would be interchangable with each other or any problems by putting say a thermal clutch on a car that calls for a non thermal clutch?