Physically, the 400 sbc looks the same as a 305 or 350 when in the car. You really cannot tell the difference. Only when you pop a head off (or pull some numbers) can you see that it is indeed a 400, because as stated above, the cylinders are siamesed, which means that there is no water jacket. The water jacket is replaced with 1/8 inch steam holes.
The 400 BB you refer to is likely a Pontiac 400 block, which is similar in physical size to a BBC. It will not drop right in to a small block bay without some mods, and it is also a fair bit heavier than a true 400 sbc.
Either of these engines is designed primarily for torque, with the long stroke. They are great acceleration engines, and the 400 sbc was originally used in heavy duty towing and RV use. If you want to compare power between the 400 sbc and the 400 "big block", it all equates to roughly the same after the extra weight of the BB and mods is taken into consideration.
The 400 small block chev is the lightest of all GM small blocks by a small margin, and it is not hard or expensive to pull nice mid400 HP numbers, and astronomical torque figures with these engines.