Trouble getting a slant six in the car

Do you have the radiator out? And leaving the front damper off the engine also makes more room on the front end.
I usually leave the transmission in the car. For a manual transmission I will bolt the bell housing to the transmission, that helps by not having to thread the transmission input shaft through the throw out bearing as would be needed if the bell was on the engine.
I leave the motor mounts off the engine. Get a couple of two inch long bolts with the heads cut off and thread those into mounting holes that bolt the block to the transmission/bell housing.
Lower the engine in, side it on the alignment bolts. Remove the alignment bolts and install the real mounting bolts.
Install the motor mounts, lifting the engine slightly as needed. You will need to free the transmission mount if you need to lift the engine a lot.
This works for me.