If this is a metering-rod controlled carb, measure/record the rod diameters (probably 2) and jet sizes, Go down 1 jet size (may not be safe for WOT) and test for engine vacuum at a specific cruising speed. If this helps (vacuum goes up), you want a rod pair with the same effective "economy step" area (jet ID squared - rod OD squared) and probably the same jet size. This will lean out only the cruising mixture.
Are you sure your vacuum advance can is working?
Some more ideas on my web page here: http://victorylibrary.com/mopar/mileage.htm