This engine may be what it says it is, but I would be cautious. The 3914678 casting number was also used on the 327 and 350, and with the last letter of the application code in question, I would not be 100 percent sure. There are 327 and 350 codes in 1968 that start with M.
291 heads are common but the 3279 crank is not common. Just my 2 cents.
If I was looking for a 302 I would buy it.