I've done this mod to a LD-340 as per the engines book by MP.
I would not recomend this for a small cam/basic street engine. It does work well once the cam gets big(ish) and the engine package is a plus 470HP like type package.
The idea behind the plenum milling is to take a dual plane and create alot of plenum like a single plane.
If the RPM fits under the hood of the car with the carb on top, I findlittle need to cut the plenum divider. If you need a single plane intake, a Torker will do.