Bill,
My guess is, the high level of detergents in the avgas removed all the varnish from inside your carb, and something inside has more clearance than it should. I would recommend you just buy the Quadrajet book and a rebuild kit at your local NAPA store, and rebuild it yourself. But with that high mileage, you might have issues you can't fix with a rebuild kit, such as worn throttle plate bushings.
Where are you in WA? There is a good shop in Kirkland (Totem Lake) that can rebuild your original, no agony involved. They have worked on Denis' Z/28 carbs, Alex really knows what he's doing..
Carburetor and Fuel Injection Connection,
http://www.carbconn.com/
Good Luck!
-Sam