steelworker as far as using flat top vs dome pistons its a matter of what kinda compression you are looking to get based on the rest of the way your motor is built...i.e. cam/carb/heads/etc...those are what your compression requirements are based on.

the 402 is just a factory .30 over 396 and right around 1971 is about the time that they started putting more smog equipment on the motors so that could be one reason for the different rating,along with the gross/net hp difference.