Were all 2v heads the 2-3 thread design?
I know we've had 2010s in here that had plugs blown out. The trucks always seem much worse, but that may be because that is what we see the most of with these engines. Plus, they I'm sure get worked much harder... a loaded E250 with a 5.4L pretty much gets floorboarded by drivers who are not the owners. They also run them out of oil quite often.
If the engine overheated (which is common due to the old intake design, if it still had one, depending on year and model), then that certainly could have made the weak spot even weaker.
This is such a common problem that I would not look for blame outside of FoMoCo themselves, and since this engine design's roots certainly started way back in the late '80s, as its debut was 1991 (Town Car), 1992 in the Crown Vic and Grand Marquis, 1994/5 in the Mustang, Thunderbird, Cougar, 1997 in the E-vans and F-trucks, the individuals responsible for this design oversight have likely long since retired and have a very nice pension or whatever.
Like I said, I'd just look for a used engine. May not be easy to find given the car 4.6L is slightly different than the truck version, and the fact that the cab companies love to cobble retired POS police vehicles together for a half million miles, they may be either in short supply OR worth a lot more than they should be.