Rebuild the 56 and put it in instead of the 54. Little better compression, non-shimmed bearings and neoprene rear main seal. But to answer your question, yes, you will get a little bump in compression and theoretically a speck more power. One other thing to keep in mind though is that the spring pressures are different amongst the various heads. 55 for example used two different pressures depending on the motor and the transmission. So you may want to check the spring pressure on the 56 head and compare to the 54 to accommodate what the factory designed for the cam.
