I'd put it on the rear-facing edge, with the axis horizontal, and the bore tangent to the pan's bottom surface. That way, it would drain nicely when the front is jacked up, and be slightly resistant to tear-off in a grounding. Don't put it in the center; put it near a corner, so you can jack up one side a little more and get nearly complete drainage.
You'll still leave a third or more of the fluid in the converter. In days of yore, converters had drain plugs; now, they mostly don't.