Tci 727 leaked.

I'm looking for a little direction as to what would have caused this.

73 Duster 360. Tci Street fighter 727
I'm just starting to drive the car again after getting the engine back in the car and I had no issue with this transmission before I had to pull the engine. After a few in town drives car was not showing any issues this was the first time taking it on the freeway after about 15 minutes of driving and taking an interchange to another road. I was getting back up to 60mph the engine cut off suddenly no warning if any issue was happening once I got to the shoulder and looked under the car i seen a puddle of transmission fluid and the long trail the car left on the side of the road. I crawled under the car seen and felt the fluid comming out if the belhousing. Tried cranking it over once and noticed it wasn't cranking over as it normally did and struggled to fire and it wouldn't run. I had changed the pan gasket the night before and checked the level before leaving topped it off off full.. I got the trans out of the car now and looking for any ideas on were to start without throwing parts at it and having the same issue when I put it back in.