75 coded engine come in a 74 dart sport originally

The only way to clear this up for sure is to post the VIN tag info of the car, or preferably a pic of the entire VIN tag, the casting and assembly dates of the block and the VIN pad numbers from above the oil pan rail on the passenger side. This will tell us all of the pertinent info to make an educated assessment of the origin of the motor and it's relation to the car itself!! Geof