My feelings on what happened? He dropped it in 4 wheel low, which doubles the amount of gear reduction, he gets into the throttle and finds out it seems to have double the power it did in high range due to the gear reduction. Within the first second of feeling the power of low gears he wants MORE so he gets swallowed by mores law which is "If some is good, more must be better"
So to the floor it goes and he doesn't know or realize now that it's in low range the engine reaches redline very very quickly, twice as fast as it does in high range. He floors it, holds it there, not realizing it's at redline, the valves are floating, he's still holding it down, a piston kisses a valve and bang!
The ball game is over. If the engine had 500 miles on it, that clears you of any wrong doing as I've never seen one have a mistake in the bottom end then last 500 miles.
On another occasion a used to be friend of mine had a nice 67 Firebird.
The previous owner had dropped in a chevy 350 and TH350 trans. The engine was a basic stocker with a 2 barrel carb. However it did run strong. It did not smoke, use oil, and it didn't have over 30,000 miles on it since the overhaul. He decides he wants something that gets better mileage so he sells the Firebird and buys a Isuzu Pup. The guy who bought the Firebird lived about 4 miles away. He let his teenage sons take the car out one night. The car came back on a roll back with 2 slung rods. How did they do that? They over revved it. The teen's father had been told by friends how his son was treating the car when it blew, he had the shifter in 1st, foot to the floor, wouldn't let up on it or shift, the valves floated and bang. Game over.