It might run OK with depending on the type of dual manifold. Most people don't bother differentiating between them, but there are three types of low flathead intake manifolds. There is the "Super" where each carb is placed directly over each group of 4 intake ports, the "Regular", which is similar to the "Super", but has the carbs placed closer together, and what I called the "Biased", that has the front carburetor very close to the stock position, with the other carburetor between it and the fuel pump. The first two types probably wouldn't like running on one carburetor. The "Biased" would probably run OK with one carburetor, providing it is run in the front position. Also, the "Biased" would probably run OK with progressive linkage, something that probably can't be said of the other two types.