I replaced both the exhaust and the manifold from Librands in one go on my 1.8 silver top Zetec engined Morgan so unfortunately don't know what difference the manifold makes on its own but I would suggest you ring Rob Wells at Librands - he is extremely knowledgable and I am sure he will be able to give you the info you need
My guess (and it is only a guess) is that the manifold will make a small difference to the sound as it improves airflow
