[Pete's 64]

Holly 500's normally have a 1/8 inch or so hole in each throttle plate.
This is to allow the plates to be at the proper position at idle in relation to the idle mixture slots on a 350 to 400 inch motor that idles at 750 or 800 RPM.
You may want to try sealing the holes with epoxy. This will allow the plates to be open further, and may improve mixture at idle. I am guessing that it is now OK going down the road, just at idle you notice it being rich.