Which car and motor are you wanting help on?
I use to install BB in early (pre 1971) B,E and A body with the tranny attached to the bellhousing and motor, on 426 Hemi motors I would install the motor with the bellhousing and clutch installed on the motor and install the tranny from the bottom
It depends on many things, some guys like to install the motors with the tranny on the K member and lower the car onto it fromm above like the factory did on the assembly lines, not me though I don't have the equipment or desire to do them that way