Nope, you will not lose a drop as long as the converter stays fully back on the tranny input shaft. Don't worry, it woun't fall off unless you tilt the tranny way down in front.

Some convertors have drain plugs. Those you can drain and leave on the flexplate with no mess.

The tranny will piss fluid out the back end after you remove the output shaft, so drain the tranny pan before you start. Draining the pan will not drain the convertor, tho.

Also....jack the truck way up both front and back. Support it safely.

Place a floor jack under the rear of the engine to support it as the tranny comes out. The motor will want to fall down in the back and might damage your fan or shroud in front, or wires and other stuff between the valve cover and firewall if you don't support it as you remove the tranny.