Probably not your problem, but one to be aware of.

I have a dummy T-400 in my 31 on 32 rails roadster project.

I couldn't get the yoke to slide on more than a half inch or so.

Got a jack under the trans pan - with board to spread the load and floor stands - and removed the tailshaft housing.
It's a short one on the big Buicks.

Found the splines loaded with grit and dry mud etc.
The trans probably sat out in the weather awhile before I got it.

Wire brushing the splines clean allowed the yoke to slide on and bottom out.

Next stop, driveshaft shop to cut 1" out of the shaft.