I've put a complete set of yokes from a 999 on a 748 streetfighter (dont ask, was at customers request) but didnt use the damper. they fit the frame and forks ok.
Mood: My Jota goes rumpety rump! & I have my wife exactly where she wants me!
They do fit as all the centres are the same except that the stearing damper will not fit as the 998 one is low at the rear for the damper to mount on.
The 999 damper also has an offset spherical ball joint that connects to the frame so a straight 998 type will not fit unless altered or an adaptor made up.
I have seen a 999 top yoke fitted to a racing 998 but it hd the stearing lock stops machined off and had no damper mount as well.