Enlarging intake with acid

You could buy a cheap die grinder and an expensive cutter and do much better.

Cut the manifold if you have to and open things up if you need to. Then have it welded back together. Try to keep it going from the plenum to the runner in the head and keep it tapering down. Air wants to take the shortest route to the vacuum source so keep that in mind. The inside turns can be widened, or in other words, make it easier for the air to take the shortest route around the turns. The outside of the turns should be kept small so that the cross sectional area can remain the same. This way you help keep velocity up and avoid fuel seperation as much as possible. That is just for starters. Don't cut it so the charge speeds up and slows down anywhere if possible. It should continue to accelerate until it passes the valve head. Just my thoughts.