Looking at the photos now I have to say that the manifold is not a very good fit to the exhaust ports and the gasket sealing area at the top and bottom of the ports is not very wide.
If you look at the Librands manifold closely you will see that the pipes have been flattened at the port end to more accurately follow the port shape.
And Librands supply a suitable gasket which, knowing Rob Wells, will have been tried and tested.
I might be tempted to return the manifold you bought on the grounds that they cannot supply the correct gasket and then go for the Librands manifold and gasket set.
Librands M16 exhaust manifoldDouble check that the current version does have the flattened tube design.
That big a disparity of shape could also affect the return pulse in the exhaust system and I would suspect the Librands system will be more efficient.
