Just curious, are you using the 307 (small) bearing case, or the 308 (large) bearing case ?
I'm with AJ thinking deflection.
True hemi 18 spline had large (30eight) bearing, and bronze bushed counter shaft,
and I believe the rear bearing in a "hemi" trans was small (307), I'll go check the "hemi" case I have, after breakfast.
thnx