I bought a dual 1-bbl manifold (modified from stock) for my truck a couple years ago from George, along with carbs and linkage, and it has worked very well. It would seem that going this route would be a lot easier than casting a complete new manifold; I guess the only limiting factor is supply of the old manifolds or time constraints of having one's own modified. But I see your goal here would be optimum flow - more than what a modified stock intake could provide?
One benefit of using the stock manifold is keeping the physical contact with the exhaust (if staying with single exhaust), because it takes a lot more time to get the intake hot with a dual carb setup. And that's from experience in coastal CA where it's not cold. And if you convert to headers, you can still heat the intake with water using the exhaust chamber.
Only thing I'd suggest on a new casting design is incorporating a water tube like the Edmunds intake, or perhaps a jacket cast in. Without a means for heat, more fuel will condense than make its way to the cylinders. What have you been experiencing with regard to this on your triple intakes?