DuPont’s book is not wholly reliable, and I don’t recommend it. He misapplies some of the prophecies to fit his agenda. He was a disaffected Trad and wasn’t completely honest in the way he presented some of the prophecies at that time.
On the other hand, Birch’s work is very scholarly, almost encyclopedic, and strives to point out where prophecies and apparitions are Church approved or otherwise to be given any credence.
For instance the quote that Birch and Dupont attributed to Augustine about a new “French” emperor is very odd. The idea of the Franks was not in Augustine's time, and I can't find anything he said about an emperor from Gaul. Not saying that Augustine didn't say that, but if this is a well known prophecy, from a many who was famous for NOT having prophecy, then it would stand out. Unless it is another St. Augustine that is not as familiar.
The idea of a Greater France Emperor is hard to find in many older sources. Some make mention of a great King, but there are some who place him in what is now Germany, Britain, and a host of other places. In short, I would be very cautious unless you have a decent list of sources.