"L" in a casting date is December; no reason that casting date wouldn't work for your car. Many Flint engines were machined/assembled within a day or two of the casting date, although a week or two is "typical". If you're taking the cowl tag date as the "build date", that's when the body shell was welded together at Fisher Body, not when the body finally got to Chevrolet and went through final assembly. If your cowl tag says 01A, the body could have been welded together as late as January 13th, and could have gotten its engine as late as January 18th or 19th.