Factory deck height meant "make it flat". So you can have anywhere from .010 to as much as .040 difference between two individual blocks. I know of one case where the deck height side to side was off (from each other) by .025". The '71s used the 340 sand cores, which had to allow for a 4.04 bore. So the resulting 4.00 bore had extra thick walls left behind. Truthfully, they are all similar until you hit Magnums. If there is a question, have it sonic checked. With bad core shift boring more than .030 over can be hazardous in later than '71 blocks.