I just am having a Scotts top mount put on my 99 640 as we speak. GKennedy has a top mount on his 01 640 also. You can do, it takes some work. I am lucky enough to have Scotts and the man who makes the arms and posts for them right down the road from me (okay 30 miles).
All it takes is a good welder who knows bikes and someone willing to cut/file the arm down so it doesnt hit the tank/ gas cap. The arm is made of hardened steel and can be cut for clearance if needed.
I intend on using the road damper too. I guess it is all personal taste....
I am usually drunk when I post so dont take it personally !
2010 KTM 990