You will ruin a perfectly good manifold. Either get a carb that will fit the manifold, or a manifold designed for the carb you have. Eliminating the divider between the planes of a dual plane intake will have no good results, and possibly a BUNCH of bad ones.
Unless you're planning to run the engine above 3,000 RPM most of the time, there is no advantage whatsoever in combining the flow from both sides of the carburetor. If you DO plan on running wide open most of the time, a stock 283 dual plane manifold will be about as worthless as a screen door in a submarine.
Jerry
