Does your harness still have that 3 wire bullet plug between the alternator/solenoid and the regulator?  It's possible that the contacts have become a little corroded and all the excitation voltage from the regulator is not getting to the alternator, or the regulator is not correctly sensing the battery voltage.  If you can find it try mating and demating it a few times to wipe the contacts clean.

Also, there is a test you can do although I don't recall the exact details -- I think you temporarily jump the field and stator terminals on the alternator while measuring the output voltage.  It should go up - forcing the alternator to put out it's maximum. 

Also check to be sure your alternator is grounded well.

I chased the same thing a couple of years back when i went to the Optima battery. I found a rebuild kit for the alternator on line that made it 100 amp and internal regulator. It uses the three wire hook-up and works well, voltmeter always reads 14V.

It is now working properly.

I replaced the high amp alternator with the new one from Autozone.  I installed by 289 Hipo pulley and the chrome fan from the old alternator onto the new one.  Also changed out the alternator harness since the old one had the clip-on attachments for the two small wires.

Started the car and immediately went to 14 volts on the gauge.  Voltage fluctuated at idle between 14 and 12.5 when I turned all the electrical stuff on.  Drove the car on the highway and it read 14 volts the entire time.  Idles at about 12.5 to 13 volts.  Life is good.  Yeah, I know.  Since I changed two things, I will never know which one was really at fault.  At this point I don't care.

Moral of this story:
I learned this lesson when I was about 18 years old and driving a 70 MACH I.  I have used the upcoming advice many times over the years but was hesitant to remove my shiny polished alternator for one that worked.  OK, so what is that lesson?

If you are having electric issues, replace the alternator and voltage regulator and all will be well.  If starter related, replace the starter.  It is cheap enough on these cars to replace all three items and barely spend $100.  If I had done that in the first place, I would not have been messin with this for three or more alternator removals/reinstalls.  The shotgun approach to electrical system repair !!!

That polished alternator case was too shiny to be under my hood anyway...

dancing in the street time, just this time keep your pants on,,  

bet everthing from fuel pump to lights will work a lot better,,  

I installed a Vintage Air HVAC setup in Al McGee's 65 fastback, and hos blower blew harder than mine.  Not any more !  Maybe this was the cause of alot of problems.  We shall see.

Now to move on to the next problem...  BASH THRASH is in full swing, and the WYAIT MONSTER is watching !!!!!!!