Did you try freeing the flexdisc from the diff-flange first, then with the center support free, working the shaft & flexdisc onto the diff flange?
It may be impossible to collapse the driveshaft enough to get the shaft's pilot bushing over the flexdisc if the disc is still bolted solidly to the diff-flange, and/or if the center support is bolted to the tunnel first.
If you decide to attack this problem from the tranny-end, the same issues may apply. In any case, if you need to remove the tranny-mount, remove it with the crossmember, after putting a jackstand under the tranny for support.

Happy Motoring, Mark

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DrDKW