I did a bunch of research before I purchased my centerforce dualfriction clutch. They can be had for about $300.
I decided not to get the SPEC or the King Cobra. Alot of people were complaining about the intermittent 'rattling' noise that these would produce.
I have a couple of thousand miles on the centerforce now. About 1500 of them with the 9psi KB kit installed and no slippage or weird noises whatsoever. I highly recommend the Centerforce clutch!
Joe