75 coded engine come in a 74 dart sport originally

"Casting" numbers indicate when the block was actually cast and the "stampings" indicate the model year/VIN. Blocks would tend to have earlier dates than the actual manufactured car. Vehicles tend to be started in summer or fall for the following model year. (i.e.: 360's were first installed model year 1971 vehicles, but there are some with 1970 "casting" dates.) If no "stampings" on the block, it could be a factory replacement/warranty block.