4.0 Mercruiser

[WHW]

I have a 1997 Bayliner with a 4.0 Mercruiser. The manifold that came with the boat was a multi-piece manifold, I bought a used manifold on Ebay when I got the manifold I discovered it was a single piece design. It appears it will fit and the gasket is the same for both manifolds. Can anyone explain why Mercruiser used these two different designs, and any advantages of one to the other. Any help would be greatly appreciated.