You need to watch the movie, point.....
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