Sounds like someone knew about this and patched it up enough to sell it to you. No idea how it didn't fly out. I'll bet they were praying it wouldn't blow out on the test drive.
The method above is what people usually say to do. It sucks to have to do it. I am putting a 2009 4.6L 2v (Crown Vic) motor in mine and I'm hopeful that maybe the 2009's have more threads on the heads.
2001 Ford Mustang GT Auto
2004 Chevy Colorado Crew Cab (daily driver)