HELP

I recently bought a 1973 Plymouth Duster V8, 4 speed manual on the floor trying to figure out if the engine is a 340?
Also there is a number stamped on the left side of the core support B3144519
i was told this car is a 73 Demon, but VIN does not indicate to be
The VIN does not decode the type of transmission installed at factory
Any help would be much appreciate it