Did you make sure the input shaft bearing fit on the input shaft??
I Had problems with transmission install too although it was out of the car ( easier ) and I had slid the new input bearing over the input shaft so I knew it fit.. ... My neighbor, he is only 63, and I grunted around for a while, even tying a strap to my cherry picker to hold the trans while we " wiggled" it....
I ended up taking the bell OFF the engine and bolting the trans to the bell and attaching it that way to the engine block..... Went on like butter... Not really sure why? One nice thing about this way, was the TOB and the fork were already on the input shaft and I didn't have to fight them coming loose or falling off......I even took a pic of the strap helping.......