[Eric]

Ok, had a look at my tranny which is all in peices. If all you plan to do is shoot the diff with oil, that boss points basically right at the ring and pinion mesh point allmost pefectly. If, again, you are just doing the ring and pinion, then it is very simple to pop the side cover off and drop the diff out of the thing,, do your drilling and then re-assemble. The whole internal spraybar setup however, is nice because it will lube every single gear, for some reason, even the reverse idler gets a shot, as well as the ring and pinion. It is just a bit more work to do. If you are feeling very lucky, you could just take the first 2 covers off, use a die ginder, magnets, vacuum and some rags to keep the shaving away, grind out the center plate and install the tube that way. The only other part that needs any work is the end cover, and it is off.
As for the suction line to the pump, not sure why they said to use the place in the pic above, i just figured, like you, that it became the drain instead.
Also, not sure if i mentioned this, and you guys probably already know, but the plumbing they said to do like this...
transmission suction line ----> cooler----->pump ---->spraybar.