Stop in for a cup of coffee

Ok I have a question. I want to eventually swap out the headers for a manifold. I have a 318 in my dart. I found these on marketplace place. 1973-1976 Mopar “A” Bodies | Mopar exhaust maniford great shape went with headers instead | Facebook
It says small block but is there anything else I should be looking for in regards to fit?
Not to be nosy but why on earth do you want to take out headers for manifold? The advantage of headers span from fuel economy, more power, engine breathing, ease of working in engine bay etc etc
just curious, your circus your monkees!