Dart w/ 727 and Vibes and Yoke Question

All,

I have a 73 Dart with a new drivetrain but has the dreaded rythmic vibration. I talked to a mechanic who indicated that my drive shaft is too short and the yoke too far out. In the below pic you can see the wear area from an earlier time. I have a new tranny and driveshaft. He recommends lengthening the shaft. Before I do that Can someone confirm this, and what is the proper procedure for Yoke installed length, before measuring again and ordering a reworked driveshaft.

Thanks,
Marion