I did this mod a few years ago. As far as trying to put a drain plug on the bottom, I don't think I would screw around with that. I'm afraid a person might make more trouble for himself then what you wanted.
When you replace the tube that's in the the tank (the one with the filter), bend the copper tube like stated in paines link, or maybe just a little more, then you will be able to "pump" most of the water out. Won't need the lower drain point then. I have a filter on the pump so I wasn't concerned about losing the one in the suction line at the tank.
I posted on this mod when I did it, along with pics.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow