I had the same problem last year. I think the problem was that the input shaft was not parallel to the floor, so moving it straight back horizontally on a trans jack was actually trying to slide it slightly upwards relative to the input shaft. The engine cannot adjust to the angle because the front is held in place, not allowing the engine to rotate forwards.
Options to resolve:
-Angle the transmission jack and just slide the transmission along it.
-drop (or raise) the front of the engine a little bit at a time while raising (or lowering) the transmission jack so you don't break the input shaft.
I did the first option and was successful, but I had someone else there to help keep the transmission from falling. I also used ratchet straps to hold the front of the engine up.
Edit, now that I'm thinking about it, I think we did a bit of both options. Lowered the front a wee bit, then just pulled. Using the front cross member as a foot brace helped a bit, but once we got it lined up properly it just came off within moderate effort. I remember the engine did twist forward a bit when the transmission finally came off.
Oh hey, I actually wrote it down with a picture. Apparently we had to raise the front a bit... Not as much detail but if you want to check what you have done against what I did here's a link.
https://www.ft86club.com/forums/show...=151713&page=3