The date code should be on a small pad directly
under and near the edge where the
oil filter adaptor bolts to the block. It is a small number (~0.25") cast into the block and will be of the format ‘7D25’ where the 7 indicates the year (1967 here), the D indicates the month (April here – Note that the letter I is not used, so it will be A through M), and 25 indicates the day of the month. It also looks almost like the head of a common screw before and after it. These were probably holding the plate into the mold. My 390 block is '1J28' (with two dots under the 1, to differentiate that it is 1971 and not 1961), hence September 28, 1971. Use a mirror and a flashlight and you should be able to see it if you can’t get under it, or maybe hold a camera under it and snap a picture.