this may clear things up for you not trying be hard on you just can not see why you went to flat tops ? is this a tall deck bottom end ? are you sure you have a 427 and not a454? the O port heads i am talking about is the head we used with a flat top 454 engine to get around 9to1 that head was a 215 101cc.or 206 97cc. now if you have a 3.765 crank with the heads you have on there now your lucky to be at 8.5 but if it was a 454 with flat tops at would be 9.1 as it has a4.000 crank now the same build in a 496 would be 9.5 as the crank is now 4.250 .same crank 4.250 as the 496 in a 548 with bigger bore4.530 now is at 10.3 .volume in bore and storke make bigger engines and the CR will go up if the head CC in the chamber is the constant