I have not purchased from this shop (yet), but Dana at Pro-Built Automatics seems to have a good rep, at least with the local B Body folks in the LA area. I spoke to him a few times over the phone and he is a very helpful guy and no pressure to buy. (TH350 page
). He's over in Upland so he is local.
I have also heard equally good things about PerformaBuilt from the F-Body crowd.(TH350 page
Additionally, I also recommend calling up Mark or Lee at the Auto Parts Firm
over in Gardena. They source their performance transmissions through a local builder who's name escapes me at the moment, but they offer good warranty and encourage heavy racing use.
Hope this helps!