I don't put anti-seize on plug threads. A drop of engine oil on the threads before installation can save your bacon at a later date if you install cheap-*** spark plugs (Chinese-made Autolites, Champions with coarsely-cut threads, etc). It is not necessary or beneficial with well-made spark plugs like the NGKs you bought, which will not stick. And in fact, NGK
wishes you please wouldn't use it; it's № 1 on their list of spark plug install tips.
1960-1974 Slant-6 cylinder heads have stamped aluminum spark plug tubes. At the top of each tube, just under its outermost flared rim/lip, is a rubber O-ring that seals the tube to the head so you won't have an oil leak. These cook to death over the years; yours are almost certainly crispy, hard, and cracked. Replace them. Info is in the list of tune-up parts and technique suggestions
here. Since the plugs must be removed to remove the plug tubes, it's best to put in the new tube seals when you change the plugs.