Custom fab is the only way - my old Mantula had headers, and Martyn (the new owner) has documented his restoration with pictures - his whole blog makes for good reading, but specifically he has a engine bay picture here - http://www.marcosmantula.com/2011/05/problem-manifold-sorted.html and I'm sure he'd be able to provide you with more info if you message him on the blog.And if you ever want to sell your Mantula.... I miss mine :(
Nick (Marcos-less in Nova Scotia)