If you find the trans output bushing to be the problem, it seems logical to me that something caused the damage, which would lead me to question the yoke & driveshaft.
Yoke engagement & spline condition, shaft imbalance, shaft runout.
Reading through this thread again, I just recalled something:
In '73 I ordered a Z28 and after it arrived, drove it a week or so before I noticed a rhythmic vibration.
Took it to my local dealer and after a couple days they found a bent driveshaft, which they replaced under warranty.
Why it wasn't immediately apparent when I picked up the car, I don't know.
Another thing that pops up is how many times someone breaks a driveshaft at the track and the output area of the trans gets damaged.