just to get you started, the 666 hood is longer by about 3 1/2 inches I think. Right side sheet for the radiator is different, 666 has a dimple to clear the lower radiator hose. Radiator is different, 656 lower hose is on LH side, 666 is on RH side and bigger. Frame rails are different also.
I have one of each sitting in the barn. Thought about putting a D312 in the '56 but it looked like too much to change.