It might not be a bad idea to see if the timing chain came loose. I had this happen in the late 60s. It backfired once when I tried to start it instead of dying while running, though. Because nobody expected the timing chain we tried a lot of other things before looking there. If it had happened while the engine was running, I'd expect there would have been noise, however. Good luck!
Jon

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end