On '68 340 engines Chrysler stamped the last 6 digits of the VIN on the top rear edge of the engine block on the drivers side. The matching number will appear right across from it on the bellhousing if the car has "matching numbers".
Exactly. Most of the 68 340 cars do have Blocks with June 67 casting dates. Even if its not matching #, its the correct engine and year. Not that big of a deal unless its some kind of show car. The vin# is behind the drivers side valve cover like in this photo from another thread.