It's not what fits the engine, it's what fits the BODY. And there are certain heads that have smog fittings etc, but those can be plugged and modified to fit. "I believe" you would not want 340 manifolds on small port 273 heads, either.
Ok I had no idea. I’ve seen long lists of what exhaust manifolds fit on 340’s and they categorize based on years but I thought they meant engine. I guess it’s body