Can be done. Word of cation being a 78 and dealing with smog law. I know one guy out in San Francisco who had that very swap done on his. Now the smog nazi won't smog his scout. Due to the slight difference in emissions between th two motors. My understanding of getting around it, for the swap, is make sure all the smog equipment for the 345 is installed correctly, then take it to an inspector for a recertification with the new motor.
Unless you live in a state or county that isn't as strict on smog legalities.