[Frank Messina]

You're on a slippery slope there. As the idle speed drops, vacuum drops which may be allowing the 8.5 power valve to open, sending in more fuel and killing the idle further until it finally quits. The only way to be sure is to go back to the 6.5, hook up a vacuum gauge and measure your vacuum at idle. There are different rules of thumb for selecting the correct power valve, but on a street car I would suggest 1/2 idle vacuum. Set your timing, then go back and fine tune the idle mixture using the vacuum gauge and set your idle speed. Float level should be so that fuel just reaches the dribbles out of the hole on the front, and barely reaches the bottom of the hole on the rear. You should be goood to go at that point.
Frank