I'm not really sure if that would hurt anything. Ie pilot bushing ect. But if you left it jacked up and tried it and it was only for a few seconds I don't see it doing much damage if any. But I'm not a trans guy. Might want to see what others say
are you sure it's not the rear end? If not I would have to say there is an issue with your input shaft. Are you sure that it's been cut down to the correct length?