Pontiac built 3 SOHC 421ci engines in the early '60s. The photo I posted earlier was the one engine that survived, and was presented to Chief Engine Engineer Malcolm McKellar upon his retirement. McKellar had it placed into his personal 1963 Grand Prix, and it's running to this day. McKellar said that during the testing, the SOHC 421 engines produced around 625 hp and were capable of turning 7,000 rpm.
What's really cool about this engine is that this isn't a tri-power setup on top, it was an an experimental single six-barrel unit that Pontiac Engineering tested for the 1967 model year. But when GM pulled the plug on multiple carbs, this setup was shelved.
Below is another view of the SOHC 421 I posted earlier, the one with the belt driven, rear mounted cam drives. Check out the exotic fuel injection setup.