Replacing engine without removing the tranny?

For my two cents, I would say leave the tranny and put a floor jack under the tranny with plywood on it to support the tranny while you have the motor out.
If you swapping in the garage there might not be enough room {Height} to get the motor and tranny in one unit.

I swapped both of mine 318 /904 and slid the tranny under the passengers side, then dropped the motor in then jacked the tranny into place much easier than trying to do both at the same time . I'm glad I did it this way , a couple of bolts are difficult to get to on the bell housing , no big deal you just deal with it !
Good luck and get some help gliding the motor in and lifting the tranny onto the tranny jack under the car.