You're not going to find much information in this section about TH400 clearance work. I would PM Craig Bush, Junkie414 and Geo2JZ, all of whom, I believe, have done this. I would also check the dyno section for TH400 cars and PM the owners about this topic as well. Good luck.
I'm running a 400, but with a J3 bell housing and ATF Speed adapter. No clearance issues what so ever. Getting the rear mount to bolt up to the mount adapter was a tight fit, but other then that it was pretty simple.