Th400s are used behind monster trucks (1500 average hp) and drag cars, I would think it would hold behind a stock 8.1 just fine. A 4L80E would do it as well, an Allison would be nice, but like was mentioned, very expensive.
And budget build with an 8.1
If memory serves, the 8.1 only got a 4L80 for one year regardless of model, after that the 8.1 became a Duramax level truck (tranny/rearend/ect) minus the Duramax itself. For a fact, anything after 04 used a Allison, and a returnless fuel injection system which I am currently cursing. 03 used a return style system if your curious.
I dont blame you on going auto, I to run autos. And I rationalized it simply, I have a due date with a hospital once every few years it seems and it usually requires major corrective surgery to fix whatever I manage to break inside my body, therefore, if my lady is out with me and said accident happens, I will need her to operate my vehicle. And besides, I only need her to be able to operate one stick, so I simplify it for her and keep my vehicles auto.