75 coded engine come in a 74 dart sport originally

Production for most of the musclecar era mopars started in early August and ended in mid to late July. So, to use your example, production for the 1974 model year would start in August of 1973, and last until July of 1974. Engines usually have casting dates that are a couple of months ahead of the build date of the car, this has to do with the casting process, getting the block from the foundry, and building the engine. I also understand that there was usually a couple of weeks between model years when the factory re-tooled.

So, long story short, a 1975 block in a '74 car most likely means its not the original block. Could be a warranty replacement, or it could just be that someone had a 1975 block laying around when that car needed an engine.

Cars often have casting dates earlier than the model year of the car, not later. My Challenger is a '72, but it was built in the first or second week of production for the year, with a build date of 8/17/71. As a result, all the parts original to it have '71 casting dates, and it has quite a few items that were actually '71 model year pieces, likely leftover from '71 production.