I'm sure it is a nice bike but $6K is way outta line unless:
1) it is cherry. I mean eat off the inside of the pipes cherry.
2) It has HRC cases (magnesium or beryllium (sp?))
3) it uses a non stock frame
4) you have all the paperwork authenticating it as a Kaz OMT bike
Kaz was not the only one with big bore kits back then. Action Fours, Henry Abe, Pops yoshimura, and RC all had kits available for the 400f at one time or another. All pretty rare stuff but not as rare as one might think.
all this basically begs the question: do you have pics of the bike?
You want rare? A buddy's uncle has an rd350 with an rz350 top end (with liquid cooling), in a custom made road race frame with tracy bodywork. The motor was way wokred and before it was a street bike Dick Mann had use the bike in a few races and he had the paperwork to prove it (photos, etc). He couldn't get $6000 for that bike. It is only worth what someone is willing to pay, and jap bikes just aren't quite there yet. Although I would love someone to prove me wrong.