I have had luck with
www.bowtieoverdrives.com a couple of times, one of them on a 750 Hp turbo LT4.. (pictures in a thread called wither turbo project)
and i got screwed by a transmission company in NY that built me a GM/AOD hybrid tranny... (tranny was fine, but he didnt send any of the 1200 dollars of extra parts i ordered... )