You will give up significant low end grunt right wher you do the most of your driving. Why not address what ever is really causing the intake manifold leak and use the Dual plane manifold.
Installing a plenum divider in the single plane manifold will not restore the lost low end power compared to a dual plane. If the motor is mild you should be using a dual plane with a fully divided plenum anyways.
Fix the intake you got. Or get another dual plane manifold.Not the cast off old junk your machinest has hanging around that he wants to unload on you. Don't assume it's the intake causing the problem. The heads intake mating flange could be out of wack also.
My Professional products crosswind intake manifold fits like a glove and makes big power. (I didn't even need to port match it to my heads) I've used both edelbrock and the PP on the same motor even.
The edelbrock RPM air gap has a more pleasing appearance of the as cast outer surfaces, but the fit and machining is the same.