Testing with my G-Tech accelerometer showed a .2 sec and 2 mph improvement in performance
over the stock intake, with no other changes. Lowering the center
divider did not hurt
low end performance or throttle response with my 406.
While I was at it why not separate the water crossover from the manifold? This accomplishes a couple of things: 1) Positioning the manifold for optimum port match is easier without having to worry about the water neck to timing cover seal problems. 2) Intake swaps at the track can be made without opening the cooling system. I used a saws-all to make this modification - took about 10 minutes. After sawing the manifold in two clean up the sawn surfaces using a grinder.
I started with a cleaned up 1970 Q-jet Intake. Modifying the manifold took me about two hours using a cut-off tool and a die grinder. If I were to do this modification again I would take the manifold to a machine shop and have them mill the plenum. If you want to do it yourself with hand tools first, following the outline of an open plenum spread-bore carb gasket, mark off the area that is to be cut. Then, using a pneumatic cut off tool, cut away the material joining the four barrel holes going down about 3/4". Using a cut off wheel makes the job go a lot faster since there is a lot of material to remove, especially on the drivers side of the manifold. Once you have cut away as much material as the cut off wheel will allow, go in with the die grinder to shape each side of the plenum and round off the resulting divider. I must stress to leave edges rounded and also, don't forget to radius the underside of the carb flange on the passengers side to improve air flow. As an option, you can start with the divider at the stock height if you feel that lowering it will hurt low end performance.
Once you are all done cutting and grinding take a sandpaper-roll and smooth everything out.