I'm replacing my own transmission. This is my first time. I didn't know until after I reinstalled my new transmission that I was suppose to put the torque converter on the transmission first. What possible damage could there be?

 

I would pull it back out and put it in the trans first. There are 2 to 3 sets of splines the torque needs to mesh with. If it's not mesh will cause damage to both trans and torque when you try to run it. Tightening bellhousing bolts may cause damage too if not properly meshed. Hopefully you haven't tried starting it yet.