does anyone know if I can use of 400 small block chevy flywheel with a 454 big block chevy motor.  They both use imbalance flywheels and harmonic balancers.  thsnks in advance  

to say what nobody else would because they didn't consider it a logical question, no. if you somehow get it fit and balanced, (you wont), you will go through a trial and error period and blow up a couple of starters. the starters case literially blows up due to minor imprefections and from having the wrong number of teeth on the flywheel. get a 454 flywheel, or have your's machined (under $150 wherei i work).  

No. They are not the same and you do not want to interchange them.