400hp is an easy 4L60E build to support by a shop that actually bothers to learn what causes 3-4 clutchpack failures. If you ask them about the things in this article
and about if they have the tools to replace the pressure regulator valve, they should be OK. BestBuy Transmissions has a good build list for their 4L60Es; IF (and that's a big if) you can find a local shop, I'd always recommend that route. Go ask the local racers, plow trucks or delivery fleets who does their transmissions. Heck, if you were close to Huntsville, AL, I could build you one. It wouldn't be cheap, but it would meet all your criteria.
EDIT - I only say that because now that the cores have 150-200k miles on them, you really have to check every valve (doesn't take long with the right tools) and have specific tools and reamers to correct internal leaks to build one that will last.
The other option is to buy one from a big name from the Summit Racing catalog, but I have no idea what's inside those. They do ask a pretty penny for them; I don't know if that's from parts, union labor, or warranty. Nor if the units are new or remanufactured.