I never heard that one. After he ran the AMC's, Penske switched to Ford products for a few years, then dropped out of Nascar entirely for 3 or 4 years, then came back with Rusty Wallace and GM products. AMC dropped out of factory support for Nascar circa 1977 which caused Penske to switch, if I'm remembering correctly. Chrysler lost all interest in doing any development on the Gen II Hemi after it was handicapped to the point of no longer being really competitive in NHRA Pro Stock. circa 1974 or 75. There were no Chrysler race teams to do any development with after the Pettys and Harry Hyde (#71) were left out in the cold by the withdrawal of all Chrysler factory support and switched to other brands. In addition, anyone developing hemi heads for Nascar racing was wasting their time as for many years, the Nascar rulebook specifically stated "Hemispherical type combustion chambers" are not allowed. I haven't seen one of their rulebooks in over 25 years (Norm might have access to one), but I'll wager that they still contain related phrasing - probably something to the effect that no cylinder heads can be run that are not specifically approved by Nascar and that certainly means NO HEMIS!
I definitely remember the Charger 500 very well. I was just relating some of this Charger 500 detail to my son. You have excellent timing
I also appreciate the picture of #71, though, thanks for posting.
I wonder if there's a way I could pose the question about the AMC NASCAR engine development effort direct to Penske - I wouldn't know how to get the question to him, since I'm a nobody and have never been in the Biz. He'd be the only one to confirm and flesh out the detail, or deny the story.