@Brian -- If the goal is 400 hp (especially at the wheels), which is a fair bit actually, I'd go sit by the nearest defibrillator and call for quotes at the places that sell kitted-up Tremec T65 conversions. That would probably give you both a slight and a tall overdrive. They also make 5-speeds with an overdrive. Some clearancing of the tunnel will be needed. A 4L60E is also a possibility if you want to stay automatic.
Looking further down the road, 400 hp from, say, a 383 stroker build of an old-school SBC is certainly achievable, but whether the result will have kinda rough and rowdy ways for a daily driver, and the likelihood of its passing California smog, are two different questions.