340 Intake Manifold Heat Shield

Hi guys,
I need some recommendations on how to remove the heat shield of my stock cast iron intake manifold. I searched the forum and found an explanation on using a chisel under the head of the stem and then unscrew with pliers since the stems have something like a thread.

Unfortunately this did not work for me so I cut a small groove and used a screwdriver -> did not work either.
I do not want to grind down the heads since I would like to keep the stems.

Thx in advance!
Best
Chris