Quick/dumb question- how to visually identify a 340 vs 360 vs 318 (with a 4bbl)?

If you're looking for a 360 Magnum ask the salvage yard if they can source one from another yard for you. They are all hooked up in one form or another. Or look for someone selling complete car with one in it might be cheaper and it will run take what you need then just part or scrap it.