You mention a "build code" of J108. Is that a casting code? If it is, that is the day when it was cast. The engine "build" for use in a specific vehicle took place at a later date.
As Pre'68 Dave posted above, that might indicate a casting date of Oct 1958 or Oct 1948 or, I guess, Oct 1938.
If you cannot easily post photos, tell us the location of the water pump, tell us if there is an oil distributor on the manifold side of the block, and also tell us the number of head bolts.
