I was thinking that was a casting date. Since it is the stamping it is a Flint built engine, built March 7. On the bell housing flange opposite from the casting number is a casting date. That should be just shortly before the build date. I would expect to see a letter B or C for February or March. More than likely a C for March since GM Central Casting, where this block was cast, was just up the road from Flint in Saginaw Michigan. An engine could have actually been assembled on the same day it was cast, depending on the shifts it was cast and assembled, but I'd expect a day or two later. That letter would be followed by a one or two digit number representing the day of the month (1 through 31). Then one digit representing the last digit of the calander year. An example would be C 4 7 (March 4, 1967).
I have edited my previous post. to reflect mu corrections.
Tom