What is your goal, and how would fabricating a manifold accomplish it better than an OEM item, or one of the aftermarket manifolds that are available? Do you have the engineering background and the shop facilities necessary to evaluate your project once it's been completed? I would think a flow bench and an engine dyno would be necessary, at a bare minimum. Do you have the foundry facilities available to make castings, or do
you plan to use welded exhaust pipe, etc. ?
One of the projects I have in the pipeline (if I live long enough) is making a 3-carb setup for a 230 engine, using Keihin side draft carbs for a Harley-Davidson motorcycle. That will require fabricating three small adapters from aluminum plate and conduit to transition from the carb bases to the cylinder head.
Jerry