What are you trying to acheive with your planned manifold? I am a little confused as to why ready made manifold wouldn't "suit " your race engine. They are in the business of marketing manifolds so enquiries from some one wanting to make their own could be considered a bit cheeky. As for the dimensions buy a gasket for a DCOE and work from that . Depending on what brand throttle linkages you use will determine the spacing between the carbs.