Happy 4th to eveyone.
FYI
The starter solenoid has been working perfectly with the lighter spring for a few weeks now. I got the silver grease in the mail but haven't used it on the connections yet because the car is running well and now is the time to be driving the car. The weather is too nice to spend in the garage working on it.
Mike.
Mike,
Great news and good to hear that you found the source of your hard starting issue. About 11 years ago I ran into a similar situation with the starter (part number 1108338) in my '69 RS (327 with Powerglide). Went through many of the same checks that you've struggled through and finally had the starter and solenoid rebuilt and since then it has been 100% reliable at all times, cold, hot, etc.
Moral of my story was that although all of the electrical connections were obviously very important to ensure minimal voltage drop, etc., the root cause of the my hard starting issue was the starter / solenoid itself. Had both rebuilt and all has been good since.
Ran across the attached while looking for something else recently and I thought I would include it here for the future reference of others. You will note, that after making a few basic checks, Chevrolet's recommendation to the dealers back in Dec 1968 was to replace the starter.
Happy 4th of July!
Richard