I agree with these suggestions. Additionally, I recommend using a frame spreader and making sure the transmission is in neutral--saves having to rock the car or turning the engine to make the spleens align.
If you’re trying to install the engine separate and after the transmission is installed you need to reach in the bell housing access cover and turn the transmission shaft while a buddy slides the engine in place.
Also, make sure that the engine is hanging at a bit of an angle towards the front of the vehicle so that when the transmission splines line up gravity will pull it into place.
