Either "4-J" September, 1964) or 4-K" (October, 1964) wold be OK. There should be an actual engine assembly date visible on the LH block near the distributor.
See attached -
The first is the engine assembly date code location (the yellow arrow, the red arrow is for head casting number)
The second is code "format" (year, month, day, assembler/inspector)
Jim
I should also mention that the "actual engine assembly date" (same format as the the one referred to above) is also stamped on the attaching surface of the oil pan on the right hand side towards the front. You need to remove the oil pan to see it.