Shelby Doug actually has it down well. The problem, of course, is that Shelby American never kept great records with respect to which engine went into which big-block Cobra. (Unlike the small-block cars, where we know the engine number for most, and they were also shown on the cars with footbox data tags.)The first comp cars weren't delivered to owners until the spring of '65, after the introduction of the medium-rise 427. Of course, magnesium hi-rise versions were an option. The first 427 street Cobras were delivered to dealers in the Sept. - Oct. '65 time frame, and from the reports early owners have given us, most 31xx cars were low-riser 427s. As stated, most 32xx cars were given 428s in a cost-saving effort. And early in the 33xx series, they reverted to 427s in a medium-rise configuration. Trying to determine what engine was originally found in which car requires going over factory paperwork for the specific chassis number, but there is no guarantee that the facts being sought will be found there. Clear as mud, yes?