AFR slowly richens at idle

Sounds like you have an electric fuel pump, here is a test for you.

Fire it up as normal, warm it up a little, now pull the fuse or power lead to the electric fuel pump so it quits pumping with the engine running yet.

Now you will be running with only the fuel left in the fuel bowls, without the added pressure of the pump.

See if this helps straighten out the idle, and see if the AFR stays around 13.5

The engine will shut down when the fuel bowls empty.

This will be a test, if it straightens out then your existing needles and seats sound like they are letting extra fuel by with the electric pump running, adding to your rich idle condition. They could be seeping just a very little bit, instead of sealing off properly shutting off the fuel when the fuel bowls are full.

Also could be your float levels could be set too low, not shutting off the fuel when the fuel bowls are full.

You were saying this is an old Holley.
Ethanol Fuels are hard on carb internal parts too if it has ever been run with ethanol ?