69' Dodge Dart Resto/Upgrade in NY

Maybe try a 360. Many more available at a smaller cost than a 340. Try calling Ontario Truck Parts to see if they have a core. Also, your machine shop might have a good idea where to get one. You could go with a Magnum, but my first choice would be an LA. By the time you put a 440 in it, you'll be spending a lot on stuff to make it fit.