For a couple of reasons...
First, the pan won't clear the crossmember and come out in place.
Second, is that the gasket material (in a tube) that you should use is supposed to be put on without
the possibility of oil running-down the inside of the block and contaminating the material.
For these reasons, Ford recommends the motor come completely out and be inverted on a stand to do a proper job.
People have done all sorts of things to avoid this, hacking the frame members and rewelding
, taking a chance and lift/tilt enough to get your hand in there to remove the oil pump pick-up (still risking a bad seal though) and even this -> Damian Frattasio - YouTube
(not sure how this works but, better than butchering your frame
It's a common problem with a dire consequence, IMHO
, bite the bullet and do it right, it's not a job you want to have
to do twice