What is the lowest acceptable voltage at the battery posts while cranking?

IMO anything less than 9 or so. Cranking voltage depends on a lot of things.

At the battery post. That would depend on if your car has points or has an electronic e.g. HEI or MSD, ignition.
At normal temperature, say 60F, with a battery ESR of 3 milliohms and a peak starter current of 600A you could see a voltage as low as 10.8 volts or so. If your battery was older, has lost some plate area and the ESR has risen to 6 milliohms then 9 volts.

I have seen cars with points start at 6.8V, yes, on a 12V system.

Ron