Hi Art,
I was just reading the Dec. '08 Octagon and saw your response to Brian Bebelman about his starting problem with his '70 B. I just went through a similar problem. Actually it sounds like the same problem because of the single click he's hearing. I first tackled all the connections and found that my engine ground strap was loose so I cleaned and reinstalled it. I even added a second strap on the adjacent bolts. The starter spinned faster so I thought I found the problem but then it happened again. I was doing some reading and discovered that my (74) car has a starter relay which I was not aware of. My '70 GT has the same one. By taking the large brown wire at C1 and the large brown and white wire at C2 off the relay and touching them together, the starter should spin if the ignition key is on. If the starter spins then it indicates the relay is not functioning. I pulled the relay out of my '70 and used it and sure enough the problem was solved.
The relay seems to fail slowly. First, only when the engine is warm. This makes you assume that it is the starter's armature as you suggested. Eventually, it happens hot or cold and then will still work intermittently.
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Regards,
-Dan