I can't find where you say AA says it is only 3/8" longer? I looked in my book and on page 26 is states 1½" longer then the stock V8 transmission. I know that the AODE/4R70W adds another 22mm (7/8") on top of that.
Examples...
C4 17"
C4 adaptor 7.375"
total 24.375
AOD 20.5"
AOD to D20 adaptor 6"
total 26.5"
So my numbers come up to 2.125"
So I go in the garage, measure my 4R70W and it is right at 21.125
Subtract the .875 that the 4R70W is longe then the AOD and you get 20.250" instead of the AA 20.5" So 1.875" set back from stock.
What numbers do you want to believe? There are several, but non are anywhere near the 3/8" that you found.