The first thread on the subject:
http://67-72chevytrucks.com/vboard/s...d.php?t=606611
And now, the rest of the story....
From pictures posted in the first thread, I assumed that the plugs in my '67 GMC were split and so I thought that I could remove them with minimum damage by compressing the split...alas, they weren't split, and the first plug came out in two pieces. After the education, I tried a different approach on the second one - again heating it with a hair dryer, getting a thin tool behind it and then screwing it out counter-clockwise, and it worked! Pictures below show the result of the first attempt and then the second attempt.