Actually, I worked on a lot of these in the 70's and there are a couple of amendments to the above techniques.

First, hammer the threaded part of the cotter back and forth until it breaks off. No need then to worry about mushrooming, and you will get a more solid blow, as well as the hole in the crank holding your punch in place. Then try tapping the crank arm close to the BB spindle a few times toward the center of the bike. That can help break the pin free. then go to FB's instructions.