Here's a link that will give you some different input on the fix for this... HERE

It should be noted that the fall away test as described in the service manual should be performed if any service of this type is performed on the wide glide front end to insure proper setup of the neck bearings.

Here's another cure that some have had good luck with...

Remove the top nut's chrome cover and the lock nut (1.5" nut), draw a line around the washer before removing. I take a narrow strip of sand paper wrapped around the tip of my finger, and sand inside the circle you drew to rough up the chrome. Be carefull to stay within the line, so that when your done the chrome cover will hide all sanding marks. Get a new washer and sand the bottom surface to rough it up. Install washer on the top triple tree, the two sanded surfaces go together, install nut and torque to 100 ft lbs. At this point you have to check fallaway and adjust bearings as needed to get it right using the serrated adjuster under the tree.
What your doing is putting two rough surfaces together and increasing the clamp load against them. They can no longer move independent of each other and your 'clunk' should be gone.

Another way to deal with this situation if you care to get into it, you can use some shim stock or even an old feeler guage of the proper thickness and shim the top tree to the steering stem post to remove some of the slop which should just about eliminate it completely as it is a clearance problem between the two parts with excessive clearance being the cause of the movement.