Wasn't the point of the sub-plot in American Graffiti, that the flat-head drag racer knew his days were numbered when the out-of-towner with the new over-head valve Chevy (Harrison Ford) had him beat until he ran off the road?
1. Milner's coupe did NOT have a flattie in it, it was, in fact a SB Chevvie
2. "The out of towner" (Harrison Ford) car was not "new" I don't remember what the newest car was portrayed in the movie, it was at LEAST 58, and the movie was supposed to "be happening" in 62, so many, many engines were built by that time, hell, including early hemis, that were not flatties