Although having the Gospels written between the 70's and 80's still gives them historical credibility as, Overman puts it:
Even using the more skeptical later dates of composition, the dates are sufficiently early for the authors of the canonical gospel accounts to have interviewed and learned from eyewitnesses to the life and death of Jesus. The presence of eyewitnesses available to correct any errors is sufficient whether the documents have the earlier dates or the dates given by a majority of contemporary scholars. (p85)There still are some historical reasons as to why they were written earlier.
Now, its a historian's job is to weigh the historical evidence against different theories to make sure that they fit, and the best historical theory would fit with most of the historical evidence. Of cause there can be some things that might not fit correctly, but you would assume that overall you would want to have as much evidence fitting together. Keep this point in mind during this series, as at the end I'll bring this back up.
Overman has a section in this chapter titled "Later dating is based not on historical facts but on a metaphysical presupposition against prophecy or against even efficacious insight." (p91). The major assumption that the Gospels were written after 70 AD is because of Matthew 24:15–22, Mark 13:14–20, and Luke 21:20–24. In these passages Jesus talks about an abomination of desolation (Matthew and Mark) or Jerusalem being surrounded by armies (Luke) and we learn off Josephus in his War of the Jews how the Romans took out Jerusalem in 70 AD.
The theory is that there was no way Jesus could have know that Jerusalem would have been destroyed 40 years after his death, so this text must have been written after the event to have taken place. That is all very well, and does sound like a fair historical statement. Events have to come after the writings. If there was no other historical evidence for this, then the Christian would have to take the text on face value and "trust" or "believe" or "have faith" that the text wasn't tampered with after the event. But Overman thinks that it "is a questionable conclusion based on a metaphysical presupposition against the possibility of prophecy, not a historical analysis." (p92)
We will see.