3500 RPM runs flat

Doesn't sound like a fuel to carb issue if its 3-4 psi while driving. If it goes to 0 at wide open throttle (or any throttle for that matter) you'll have a few seconds before the carb runs out of fuel. Been there, done that. :)
6-7 psi is the 'cutoff' pressure, in other words the mechanical regulation which only comes into play when the carb's inlet is closed or nearly closed. 3-4 psi while driving and accelerating is typical on a stock type pump.

I think instead of guessing with you, we should try to help you diagnose it.
First, help us understand the conditions this occurs. Does it happen revving the engine with gear box in neutral? How about cruising in 3rd on highway? Or is it just at full throttle acceleration?

Next, take a look over each of the systems to check condition. In other words, diagnosis will be isolating the problem to one of three things needed to fire a cylinder; spark, fuel+air, compression. I'm assuming for the moment this only happens under load, so unless you have a shop with a load dyno and oscilliscope, the best way to start is look for clues.

So look at these items:
Remove and check spark plugs (take picures if you need).
Check the ignition wires and inside cap and rotor. Look especially for arcing or carbon traces or moisture.
If the coil is labelled, check the resistance to specification from manufacturer

If you have a timing light and tach, check the timing curve. (remove the vac advance hose and plug with golf T or similar. Then measure the timing from idle to 3500 rpm. Write it down! Record every degree or every 200 or 250 rpm or whatever is convenient. Re attach vac line and note how much the timing increases at idle vac)

Holley carb:
check fuel level (sight plugs) on primary and secondary.
Check choke is fully opening when engine is warm
That's all for now. After you answer the questions on when it happens, we can decide if it is worth looking at the primary main circuit to see if its clogged up or way off or something.

Engine: If you want, check compression dry and with oil. Or if you have the inclination, a full leak down check. And yes shure you could check valve springs but I think its best to eliminate possible spark or fuel issues first.