Another good source is F14CRAZY. You'll find him on GM Inside News Forum
as well as here and a couple other places. He actually put in an F40 6-speed transmission from a Pontiac G6 / Saab 9-3. It bolts right up to the engine, but you need to fabricate all mounts and the interior parts.
The F40 is a really good candidate because it can take up to a 300 HP / 300 TQ engine, but keep the car below that to use the stock parts. I'm actually planning on using the F40 in my turbo Bonneville setup, but I'm going to need to get a lot of custom parts. I'm sure there are basics you can upgrade, like shafts made of tougher metal and a tougher chain setup. That'* probably going to give you some head room to get to 400 HP/TQ "safely." Of course, you will need a custom flywheel, clutch, and so on. See which places F14CRAZY recommends.