I think the first week of March is too late, the heads were cast in Cleveland Ohio machined and the engine built then sent to San Jose (Milpitas, CA) to be installed in the car, two weeks seems a bit tight. (then sent to Shelby to be completed).
Heads are date coded, it is under the valve cover.
Image added, 6B25 = 1966 Feb 25th - as an example
also note for HiPo Heads screw-in rocker studs and recessed pockets for valve springs as well as other molded-in identifiers.