OP; don't get mad at me, but since you haven't told us why you want 400hp yet, I'm assuming it's NOT to go racing, so that sorta leaves it being a streeter. OOPs I missed it post 9 says "driver"
If that's true then it seems like most guys are quoting the 400hp number, which in a 340 may not come until pushing 6000 rpm. But what they really want is a smoking hot bottom end starting right off idle, or at least by the stall-rpm. Midrange is gonna be similar no matter how hot the street engine gets to be.
Now not to belabor the point, but 6000 rpm with 3.23s and a regular Torqueflite is about 61 mph, if not spinning. But if you are running street tires, they will be spinning at WOT for the entire duration, if the engine is assembled and tuned right. In fact, even 350 or 300 will do that. Maybe even less. So why spend a boatload of cash to get a 400hp@6000 rpm engine, versus a 350 at 5500, or a 300 at 5000..... all of which can smoke street tires, on the street, given a street rear gear and a streetable Convertor.?
400 hp is asking for 3.73s minimum and a stall as high as 3600
350 is around 3.55s and 3200
300 is around 3.23s and 2800
250 is whatever is already in the car, saving you another boatload of cash.
Point being;
what makes 400hp the magic number for you? and
what rear gear/trans/convertor are you thinking if running.