John, in autocrossing (also known as parking-lot racing), I got a backfire on the starting grid with a friend's Pantera that wasn't sufficiently warmed up. The 351-C had been rebuilt with 11:1 c.r, bored 0.030" over, big solid lifter cam & Holley carb, and needed 102 octane fuel to run without detonation. Immediately after the little backfire, water was seen running out one exhaust pipe. A teardown showed a piece of cylinder wall about 1" x 1" had cracked and blown clear into the water jacket. The retrieved piece showed a cylinder wall thickness of only 0.050"! For performance use, cylinder walls need at least 0.150" and more is better! Chev guys like to see 0.200"- no 351-C ever had such thick walls when new!
Lesson #1 learned: do NOT bore a 351-C AT ALL unless you first run a cylinder wall thickness check. This block had a shifted casting core on that cylinder.
Lesson #2: As you found out, stock valves are made in two pieces and WILL separate at the head-to-stem weld if overstressed. And 'overstressing' is ridiculously easy on a completely stock 351-C. Air-cooled VW valves are made the same way. Use ONLY one-piece valves which require single-groove keepers.
Lesson #3: I also suggest a fully baffled 10-qt oil pan or you run the risk of wiping out crank bearings from cornering forces. A friend lost a connecting rod after only a dozen laps of open-track road racing with a stock pan full of oil. Good luck on your next 351-C engine build.