You need a beefy, well attached mast head fitting that can support full downhaul pressure and a halyard with minimum stretch. Many mast head turning blocks are only designed to take the pressure of pulling the sail up and could break from 8:1 downhaul pressure since normally, the halyard lock takes over once the sail is up.
+1 I know the mast cap which held the halyard on my N20 was somewhat flimsy and most likely wouldn't hold up to the kind of force needed to keep the cunningham tight..
