That shaft looks brand new! It's a shame that it sits around not being used. Let us know if you ever plan to sell it - you might start a bidding war!
The measurements are great, and confirm what I already thought, and that is that the "business end" of the shaft is a little shorter than today's 1000 shafts. You have about 2.5" of usable spline length, whereas the shaft on a TW has 3" worth of splines.
My thought was to take a TW shaft to a machine shop and have it turned down and re-splined to fit in the 6000.
To anyone else: I have the factory print spec for this shaft. Let me know if you'd like a pdf copy.