I ran into the same problem with my QT-16. The original driveshaft is not rebuildable. I bought a used one a few years ago for $100 but it had a fair amount of play as well. Eventually it was time for a repower which rendered the original driveshaft obsolete. I then built my own from tractor supply PTO shaft parts. Here is a picture of the pieces:
The advantage to this shaft is that the u-joints are replaceable and the driveshaft is 100% smoother than the old one. The only thing I still have to work out is the fan installation for the hydro. So far though, it hasn't been a problem.
Looks like a great job, and the updates about the cooling fan were very encouraging. Did you use the 6 series parts? Your photo shows a yoke-tube assy and a yoke shaft assy . Did you feel this would be better than just the yoke-tube assy and the weld yoke of the same size? As I have an QS and two QT (one needing repower & shaft now)This info greatly appreciated!! I may use a kohler K341 I have available for the repower if I can adapt the electric pto clutch from the onan to the kohler?? Thanks for great info!!