before putty knife I'd use a 4x4 block of wood and big hammer to try to shift one end of the head sideways. As soon as it moves any it'll be quite free from there on.
If resorting to the putty knife put it between the layers of the head gasket and straight in. However you do it you will risk nicking or raising a burr on the block or head. Could be gently dressed with a file if you do, not ideal though.
Grigg
