Paul,
The total number I've heard from a "reliable" source was 44, but I don't know where he got that number so I can't verify that. Nor do I know if that number includes the first MKII which was 396cid plus the MKIIs (stroked) to 427cid. I've spoken directly to Dick Keinath, who was the head engineer who designed it, and even he does not know how many were made in total. The design of that engine never really stopped until GM pulled the plug and told everyone their job was at risk if they did any more with that engine. So, there were several blocks and other parts that were still in a design and test loop. It's possible some of them got out the door too, so it's very difficult to determine a real total. I'm sure some of them were probably left there then scrapped when they turned the lights out.
I know of 6 that exist today. In the heyday of '63 when they appeared on Nascar tracks, Ford complained and Nascar forced Chevy to give 2 of them to Ford, but I don't know who got them, or if they've ever turned up. Once the higher-ups in Chevy put a ban on racing activity, the engines were to be returned. I'm guessing that's what happened to most of them, but a few got squirreled away...
Dick Keinath was an engineer ahead of his time then. He had ideas to further test the combustion efficiency of each cylinder by analyzing the exhaust gas, but that technology wasn't invented yet. He had planned on compensating deficiencies by either changing individual port design or with variable cam timing for each cylinder or by variable spark timing (which also was not invented yet). He went on to head the team that developed the MKIV, which we all know turned out pretty good!
Verne