Too much torque

Most factory 4 speed cars have a larger u-joint/yokes than the 727 auto cars with the acceptation of Hemi and some others, but the splines are the same. Why yours broke could be age, bad part, or abuse. The u-joint breaking and high rpm could have just wiped out the yoke, the yoke may not of failed but was ruined in the wake.

There are other ways to control the torque with the turbo by tapering or controlling the boost.