That engine wasnt made in 1968 and thats not a dealership install!
Thats a Nissan SD33 diesel engine. Years ago Chrysler had a marine division and they used those engines in Marine Applications (boats). Putting the Chrysler stamp on the valve cover. It appears to be a Chrysler Marine engine in the car. In the 70's International Harvester used the same engine in Scouts I believe it was. IH used the 727 Chrysler transmission with their own bellhousing. Its not a power house diesel engine like we are used to seeing in the current pickup truck market.
To sum it up its a boat engine with an IH tranny stuck in the Dart.