Not OPs issue but I’ll throw this out for others following,
I had a nagging vibration at speed, tried everything under the sun, twice.
This was after swapping in a 518, turned out there are slight pitch differences in the 727 and 518 yoke splines.
I swapped in the 518 yoke and it cured my issue.

So for others fighting a nagging vibration, worth looking at the transmission yoke.
I had the DS balanced at speed twice and it never occurred to me to change the yoke until another member here PM’d about his experience.