Hey May70, you posted some pretty good pictures of your engine bay. All the correct angles, and in focus. So thank you for that.
That's a nice setup you got there. I can definitely see the potential.
There is one comment/question I have. And it may possibly be the root of your issue that seems to be "baffling" us all. No pun intended.
I noticed in the first picture you posted of your engine bay that you are running a PCV valve from the rear of the valve cover down to a direct manifold port. Which is correct and a good idea BTW. You are also running a sealed oil fill cap in the front of your valve cover. Again, good idea. The possible issue/problem that sticks out to me is your valve cover side draft tube. It appears you have it connected into the stock air filter. There are no other pictures you posted with the air cleaner Present after the first picture that shows it to verify.
Heres the reason why. When you run a PCV valve you need a fresh air supply to facilitate the process of Positive-Crankcase-Ventilation. In the case of a V8 engine this is normally accomplished by having the pcv valve in one valve cover(usually drivers side towards the front) and the fresh air intake on the other(usually passenger side towards the rear.) This is generally speaking and there are exceptions to this rule of thumb. Like with the inline 6 cylinder engine. There is only one valve cover to connect to. Having a sealed oil fill cap basically removes that orifice from the equation. Leaving the rear valve cover opening and the side draft tube opening. My dads 250 inline 6 that pulled out of his 64 c10 in order to put a 292 in its place was setup almost exactly like yours except for one difference. The valve cover draft tube on my dads engine had a short metal tube similar in diameter to the one you are running. BUT, the one on my dads engine was only pointed towards the rear of the engine and had a small rubber tube about 3 inches long that was open to atmosphere. No filter. Why? I dont know. When it came time to setup the PCV on my dads 292(about 2 weeks ago.) I had and still am having a heck of a time trying to figure out how I'm gonna get a fresh air supply and a functional PCV valve with a sealed oil fill cap and NO SIDE DRAFT TUBE IN THE VALVE COVER. The 292 he picked up from a guy wasnt equipped with one of those type valve covers. Not a big deal. But it was in better shape than the one from his 250. So I went with it. Any who. From what it looks like with your setup is that your pcv is connected to manifold vacuum and your oil fill cap is sealed and the side draft tube is connected to the air cleaner and your air cleaner is the stock type with the snorkel. To me, and I could very well be wrong but it seems like you're choking your engine with it's own connections. Your only source of fresh air is coming through that narrow snorkel. And your side draft tube is probably losing the battle for fresh air to your carburetors vacuum demand. While your PCV valves manifold vacuum source is also demanding a portion of the available airflow. So, I can see a situation of a limited air supply with a high demand for air. Also take into account when the throttle blades are opened above idle speed and your distributor advance adds to the vacuum effect. I can see why your engine creates such a high vacuum signal then drops to nothing and literally chokes itself out. This is purely my observation and I cannot say without doubt that I am correct in my observation. Maybe someone else can chime in...