I can't remember, is the transpak the one that you just have to drop the valve body? If so, it's pretty easy IMHO. Just follow the directions to the letter, use a well-lit area, and have plenty of brake parts cleaner on hand to spray the nasties off the valve body and out of the pan!
What kind of trans are you doing this on? I put a shift improver kit on my AOD along with the rebuild and some other custom mods I did, and after 3 years still bark 2nd gear with 3.73's.
Just waiting for something to go boom so I can get a 4R70W.