Old hat...

Ok I'm thinking............... I've never had any experience with a thing like that.
But I have this thought; Air flows from a high-pressure area to a low-pressure area. That means speed is creating a high pressure area at the mouth of the scoop. Since you have proved that air is actually rushing into the scoop, it has to be going out somewhere.
You said you hear detonation at 3000rpm in 4th gear, as you load her up. The math points to 53mph with 27" tires.
At this time,(3000rpm/53 mph), about the most air your 345 can draw at WOT is mathematically 300cfm at 100%VE. So lets say 180/200 as you roll into it, and 250 as the secondaries open. My thought is that most of the in-rushing air is going around the carb. I gotta wonder if air is still able to enter the engine compartment thru the rad.
What if the air pressure under the hood was greater than the pressure in front of the rad? That's a scary thought. You might have to install a chin spoiler....... to help Ram-air slam thru the rad, cuz at 3000rpm that flex-fan is probably doing squat. Rev it up sometime and see. I hate those things.

What is the engine temp gauge saying at this time?