I have an auto in mine and love it.
It is pretty competitive on the track, but the purists will hate it.
The 6L80e's are a beast of a box that will take what ever you can dish out but are huge. Mine is a 4L65e which has been significantly modified to handle the abuse I throw at it, a stock unit will die on the track. My first two did
Expect to need an aftermarket stall converter, mine is a 3000rpm unit still with a lockup clutch.