As a counterpoint, it's not that difficult to separate engine and transmission in the car. Does the same manual tell you to remove engine and transmission to service the transmission?

If you decide to leave the transmission in the car, don't forget to support the transmission with a jack since the engine provides forward support for the transmission. The downside is you can't roll the car with a loose transmission.

You have to release the torque converter from the flex plate, detatch transmission cooler lines (have to anyway to remove the engine), release the dipstick tube from the cylinder head, release the bowden cable from the throttle linkage and of course release the transmission from the engine. No need to drain ATF (but cap the cooler line fittings) or detach shift linkage and other transmission connections. Don't count on a loose transmission holding the rear wheels in Park!

Sixto
87 300D