I can likely find you an antenna picture in the parts book. I am pretty sure there is a brace near the bottom of the tube. As far as access, I was in a Pontiac dealership in those days and I seem to recall the guys dropping the inner fender, yes. We did lots of warranty work on those.
For the tach, I can't help I'm afraid. I'll look to see if I might have a service manual for it.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars