Ayuh,..... Check the gasket surfaces for Flatness,.....
I once found their manifolds to be .020" / .030" low for the exhaust rim, vs: the outer rim on a set of manifolds,....
Decked 'em at my local machine shop, 'n no worries since,....
Used both Merc, 'n aftermarket risers on 'em, always with Merc gaskets though,.....