Hi jimmyjam,
The "duo plug" will get you about 5 of the 13 quarts.
The rest is in the torque converter / lines / cooler / radiator and rest of the system and is very hard to replace.
What everyone does, is replace the 5 and the filter, and then tap into the system and pull a quart at a time out and put a new quart back in via the inside "duo plug".