Not on a y block. C4 mounted to an Aussie Ford 250 Crossflow.
I need to unhook and slide back the trans to replace engine rear main seal. Can I leave the atf in. Engine is in an Econoline pick up.

Assuming you leave the driveshaft slip yoke in place or use something else to replace it so that the oil does not run out of the tail housing, then you should not have to drain the ATF.  Also attach something to the front side of the converter so it does not inadvertently slip forward while you have the transmission unbolted from the block.

Lorena, Texas (South of Waco)