I had this same problem many years ago.
You did not mention what heads on the engine.
15 years ago I built a 340 with w2 heads.750cfm Holley
First few runs on new motor I had backfiring and poor performance.
Plugs were white. I kept increasing jet size and got all the way to 99 jets all four and plugs still white.
A good friend of mine told me to take my carb to a carb specialist in Hamburg New York.
The guy asked me my whole car combination.
When he gave me back my carb the car ran its best ever.
I checked the bowls and he had 68,and 69 jets and plugs had perfect color. The car b guy told me that a stock street Holley 4150 carb does not have a proper fuel curve for a cylinder head that can flow that much air. He drilled passages and all that in the metering plates as that is where the fuel flow restriction was.
The only a/f ratio reading I would trust at the track on your gauge is the wide open throttle one because that is when your likely able to hurt the motor if it's lean. Today you can buy hp4150 etc that have a proper or closer to proper fuel curve. I would try another carb to see what you get. Imho.
