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Having never experienced vapor lock before, the symptoms were a bit confusing to me.
It was a long hour in traffic with a 3 speed in MI on the way home. At one point she died & it was 20 min before she re fired. After things finally started rolling at interstate speeds, it was a couple of miles before the stumbling & popping started. It seems like all that cool air should have helped?
I was able to coast to a rest area where I blew out the fuel filter & played with the timing (The distributor was left slightly loose to allow for tuning if needed) By this time things had cooled off? And it ran great for a good 2+ hours. Up until Milwaukee County where an upside down Santa Fe had left parts all over 3 lanes of interstate. It was an hour stuck there and it was only in the mid 70's at 10 PM. After we got rolling again, it was a good mile or 2 and the bucking & popping started again. This time I got to a gas station & had a Slurpee, things were good after ? an hour.
There was fuel in the filter & the sediment bowl but no fuel shot from the accelerator pump.
Can I assume that the fuel pump runs cool down low on the engine, right by the fan? I have no PS or Air to heat things up. New water pump. Also the cross over in the intake manifold is blocked on one side to stay cool. At this point, I envision wrapping the fuel line & sediment bowl with any one of a number of products.
I am I simply new to the vapor lock game?

Just a couple of ideas I've learned from another car, Your ignition coil could "open" up when hot, works when cools off. Also if the gas cap is plugged up it won't suck air.

I've recently had similar symptoms on a car that I acquired from an older gentleman. It would run great then just sputter out after different drive times. It turned out that the fuel tank had been cleaned and sealed in the 90's and the ethanol gas caused the lining to peel off. Flakes would float around and occasionally cover the intake tube while suction was applied. After the car would die, and the suction was relieved, the flakes would float away and the pump would pick up fuel and fill the carb again. Since you' e indicated that the accelerator pump is dry, i am assuming that the carb bowl us empty and I'm wondering if the pump is starved.

I also had a similar problem with rust flakes in the tank of a 35 Plymouth and a 51 kaiser with the integral fuel pick ups in the tank. When the car went up hill for one and down hill for the other, the rust flakes would shift and plug up the pick up and then fall off after the car died ( they were too big to actually go into the tube).The cars all ran great when the obstructions moved.

It may not be related, but just thought that I would relay the experience.

With advanced age of these cars, I am now running new steel lines from the tank to the fuel pump. It only takes the most minute of pinholes to create an air leak that will mimic vapor lock and I have seen many cars with fuel lines badly pitted under the clamps. Add to that a variety of electric pumps and multiple inline fuel filters held in place with squeezy clamps and its a wonder the mechanical pump can bring up anything at all.

Remember vapor lock is always on the suction side--the pump draws in a bubble instead of a shot of fuel.

On most Packards the fuel pump is only slightly above the level the fuel could reach by siphon effect alone if there was nothing to break the siphon. I have had very good results by spending $25 bucks on some 5/16 tubing and a couple of brass unions.

As nod to ethanol laced gas I no longer run the fuel lines over the frame near the steering gear as Packard did, but along the bottom of the frame in that area keeping the fuel away from the heat of the engine and the blast of the fan til the last moment when it needs to reach the pump. This works well.

If you do a search there should be several threads on vapor lock and it's suspected causes and some possible remedies.

Along the lines of Ross's explanation another theory that seemed to be consistent was todays gas is more volatile so the fuel bubbles easier. The vapor lock symptom is a result of the lower pressure of the fuel in the suction line before the pump making the fuel easier to release gasses and "breaking" the constant flow of fuel to the pump. When that happens the fuel pump has a difficult time maintaining constant fuel delivery and pressure to the carb so engine becomes fuel starved. When the engine quits, the fuel pump quits pulling so the low line pressure is eliminated and fuel has a chance to become a solid column again. It sounds good anyway because it would seem if the problem were happening in the line after the pump, the heat soak of a hot engine should make the problem worse, not better after it sat for several minutes. Even wrapping the line was said by some as not very effective.

Several reported having an electric fuel pump near the tank and by turning it on the pressure was restored in the line so it was harder to for the bubbles to escape so the electric pump all but eliminated the problem. If all the other suggestions are eliminated as the problem and you have no electric pump it might be worth a shot and would probably do as much good as wrapping the line after the pump -- maybe more.

Mix race fuel in with the pump gas. 5 gals race fuel 110 and 5 gals of pump 93 octane. My Dodge Dart did this bad last winter and I wrapped everything and the fuel lines are new and the gas tank too. Fuel pump new and carb checked and new.

Minute I added the race fuel it stopped. Car ran perfect and the exhaust smelled great!

Now that summer is here in Texas I'm back on regular pump gas 93 octane and its running good again. Come winter I'll mix race fuel again. It's the winter mix that causes the problem. It's a high volatility because it has a higher butane percentage. Bastards.....

Chek gas cap to be sure the vent in gas cap is not plugged.
Chek the line from pump to carb to make sure the line IS NOT TOUCHING THE ENGINE!.

I assume from your description it is a V* since u daid the exhaust cross over was "blocked off". WHAT type of material was used to block it off?? Has the heat riser on the left exhaust manifold been removed or make sure it is in the OPEN position.

Except in really strenuous conditions (which I don't think you experienced) the V8 Packards seem to be quite immune to vapor lock; I can attest to that from being in stop and go (mostly stop) traffic on I-78 in PA in 100 degree heat for nearly 3 hours; no problems nor did the other V8 in caravan with me and based on that I'd first pursue Ross' suggestions.

To determine vapour lock or other if some other problem not related to fuel:
Next time it happens OR when u shut off the engine IMMEDIATELY AS FAST AS POSSIBLE lift the hood and watch the clear fuel filter bowl.

Look to see if the fuel (after about 3 minutes) evacuates the glass bowl and then after about maybe another 5 minutes fills the bowl again.

If the glass bowl stays at or about the same amount of fuel after say 10 minutes then the problem is not likely vapour lock.

What's lacking on this vapor lock stuff is some good data.

I would hook up a fuel pressure gauge and bring it onto the hood or through the firewall.

When the darn thing acts up look at the gauge.

I think the gas tank debris story is very good. I had that problem with Paul Lamb's 34 Su8.

One of the favorite bugs of ASE, GM and Ford Cert testing is falling oil pressure that recovers when the motor is shut down - reason? Debris clogging the oil pick up.