[T Bird]

Marc
When you balance vacuum side-to-side, you must have the linkage disconnected.

I presume that you have balanced vacuum at each carb--that is, with the linkage disconnected, you have each barrel of the right carb pulling equal vacuum at idle speed, and each barrel of the left carb pulling equal vacuum at idle speed. The vacuum reading for the right carb probably does not match the vacuum reading for the left carb. In order to get both sides to pull equal vacuum, you must keep the linkage disconnected (disconnect each press rod connecting the crossbar to the throttle arm of each carb). Now, slowly turn in the idle stop screw on the carb that pulls less vacuum until the vacuum reading increases to match the other carb. Get them equal. Where is your idle speed now? If it's too fast, slowly turn out each idle stop screw. Keep at it with the idle stop screws until you get the proper idle speed with vacuum balanced.

Now reconnect the press rods. If they are either too long or too short, they will exert preload on the throttle arm and screw up the idle balance. If that's the case, with the press rods connected, loosen the lock nuts on the press rods and twirl the press rods back and forth between your thumb and forefinger. You will see the rod get longer or shorter as you twirl. You will also be able to feel the middle spot between extremes, where the rod is just the right length. At either extreme (i.e., too long or too short), the rod will be difficult to twirl, because you are fighting the resistance from the throttle arm on the carb (i.e., either trying to open the throttles, or pull them shut). When you find the sweet spot between extremes, lock down the nuts on the press rod and repeat for the other carb. Measure vacuum again to make sure that you are now balanced.

Check side-to-side at 3000 rpm next, obviously with the linkage connected.

What are you using to measure engine speed? The tach on the dash or an external tach?