I'm going through this exact scenario as we speak. I purchased a TKO600 setup and decided it wasn't going to handle my recent build. As you can understand, plans change. 555 BBC w/ twin 76mm turbos is just too much torque for the TKO600.
I purchased my kit through Keisler.
For the 4L80E. It really depends on your needs. I prefer a company that can build to suit or has multiple levels of upgrades. They generally know what parts need upgrade and at what power levels.
I'm going through
http://www.ckperformance.com/ for a Stage 3 unit. Chris is very knowledgable and has good references. He knows them inside and out. I spoke with alot of "OD Trans" companies, I was a little disappointed with their general lack of knowledge. Most of them are just sales folks on the phone. The other company I spoke with that gave me a good feeling was....
http://www.phoenixtrans.com/html/gmtrans.html