Holley/Demon Carb stumble

Looks like you’re on to trying a different body.
Good move.

Even on a big block the 1.56/1.75 downleg 4150’s can be a challenge to be good at the hit.
On a 400” motor with a modest converter, it’s really just the wrong carb...... especially at altitude.
The fundamental problem is the lag between the pump shot and the full activation of the main circuit.
The main circuit is activated by air flowing thru the Venturi and boosters.
Less cubes is less airflow...... and high elevation is less dense air...... a double whammy.

I would have expected to need way bigger squirters to use that carb on that motor.

My “good” carb is a ported old school Holley 850 annular, and even when I ran it on my 448 with 5300 stall it used like 40/40 squirters on it.
When I ran it on my 340 with a 4900 stall I used 45/47’s.

The old Holley 1.56/1.75 downleg 850’s came with a 50cc rear pump........ and on most combos....... they seem to need it.