I don't think you'll find any rebuilt poly 318's anywhere. Years ago one of the Mopar resto places in NJ had a pile of NOS shortblocks that they pulled the steel cranks out of and sold cheap.
Swapping to a 383 requires a different transmission.
Putting in an LA 273/318/340/360 is pretty easy, Same transmission, same motor mounts. Some exhaust mods will be needed.