You'll want to take off the cover that houses the clutch cable-clevis connection.
That little sucker has a lot of force on the split end, and I've found that if it starts to spread out at all, it will clunk as described. When it gets a little more spread out, it can stop disengaging the clutch altogether. Best to remove it, take it to the vise, and peen the rivet back over the open end.
It's not the best design...
As far as the slipping goes, I'd disassemble the ramp and ball system, along with the little steel ball bearing (don't lose it!) that resides in that same cover. Clean it all, grease, then reassemble.
The clutch may still slip, but it's best to first make sure that all of the potential things that can hang it up are serviced first. There are a lot of moving parts involved, and they do get quite nasty over the years. It wouldn't hurt to make sure the cable itself is totally free-moving as well.
If it still slips after all this and a proper adjustment, well, enjoy the new clutch...
All the best,
Shane
