How does the metal transmission return coolant line connect to the transmission?

Yes it connects to the outside of the trans. NOW, this line can be spliced with a brass compression union i think it is either 5/16 or 3/8 od line size, can't remember. Another method is a piece of HD trans. hose and two clamps, just splice the line that way. Another way, IF this hole is close enough to the radiator, just use a longer hose to cover the hole and then clamp it. You do not have to replace the whole line!!