I called my LHS and they told me thatg the manifold would be fine. They said the following:

Although the outlet of the engine is wider than that of the manifold, if you look inside the engine, the hole in the piston where the fumes come out is actually no bigger than the hole in the manifold thus there will be no problem with the fumes trying to escape..

It is only a problem if the hole in the manifold is actually smaller than the hole in the pistonn, which you can see by looking through the outlet..

What do you think?