69 Dart Hard to Start When Hot, Starts Fine Cold. Vapor Lock?

I have been running 87E10 in my 11.3Scr 367 since 1999, with zero percolation issues whatsoever.
But then, the daytime ambient temperature around here rarely gets to 100*F and if it does, then never more than for a few days in a row.
I'm not saying that the fuel in the bowls doesn't evaporate after a few days; that is a different issue.

Here's some help;
As you probably know, our gasoline is composed of several different compounds. The lightest of these boils at around 95*F and the heaviest at about 450*. But those numbers are at a specific ambient atmospheric pressure.
In your fuel-line with the car running, the fuel pressure should be over 3.5psi. This will increase the boiling points of all the compounds. How much I don't remember.
When you shut the car off, that pressure should remain in the line until the float drops due to evaporation, or percolation in the Hot bowl. Thus the fuel should NOT boil in the line for quite a while. If it boils as the car is running, measure the pressure between where it is boiling and the float-valve. My guess is that the valve is not closed.
But if your fuel-pump is sucking air, and it gets trapped in the filter, then that bubble of air will act like a spring because air is compressible. OR
Consider that the fuel, if it is being sucked up by a mechanical pump, that fuel may be under less than atmospheric pressure...... and so it would boil at less than 95*F. OR
anytime the fuel is required to pass over a sharpness, in the line, pump, or filter, the pressure will again be reduced and bubbles will form.
For these reasons I never run a filter between the pump and the carb, and specifically; never run a plastic filter which is flexible, and itself acts as an accumulator/unloader. On my car I have a lifetime metal EFI filter canister, back at the tank. It was new in about 2002. The line from the pump to the carb, at one time, I fabbed up from one piece. But eventually, I split it right at the Dual feed.

Now, your mechanical fuelpump has valves in it. Some of them open under vacuum to pull the fuel forward from the tank, and others open to release the fuel upstream to the carb. These guys are just like your heartvalves, pulling and pushing your lifeblood around in a circle.
The thing is, that as long as the pump is below the liquid level in the tank, the pump is gravity-primed, so one of those valves could be defective and you might never know it. ..... until you shut the engine off, and the fuel-pressure instantly falls to zero as the fuel backs up in reverse, back into the intake side of the pump. Of course with the loss of pressure, the gas boils.
Most guys know that a fuelpump has to make pressure; not so many know that it also has to make a vacuum, and more importantly, the pressure must not bleed backwards thru it.
To vacuum-test your pump;
I recommend that you remove it and clamp it in a bench-vise. Then install your vacuum gauge on the intake side, and pump the lever until it can no longer increase the vacuum. The vacuum should increase with each stroke until it peaks, and more importantly, it should NOT decrease during the test AND should hold at peak for quite some time after you quit pumping; the longer the better. Sorry; I cannot specify a time, as I have forgotten it.
To pressure-test your pump;
Install your vacuum/fuel-pump tester on the pressure side and again stroke until the pressure peaks. When you stop, the pressure should remain at peak for "quite some time". Again I have forgotten the spec. I'm gonna guess several minutes at least.
If your pump fails either of these tests, I would replace it.
While you are at it, replace the plastic filter with a metal one, and install it according to the arrow on the side of it. If I had a filter on the pressure side, I would orient it vertically. The fuel line on the pressure side, should NOT be attached to the engine anywhere unless the attaching-clamp is insulated and NOT tight to the line. The engine is at ~200*F and will transfer it's heat to anything it comes in contact with and fuel loves heat. Keep the line away from the hot exhaust, which typically runs from 350 to 800*F and could run as hot as 1100. I run my 3/8 steel line inside the front stub-frame, beginning at under the front seat, and it comes out right by the pump, where I have a very short rubber jumper to isolate it from the Hot pump.
Like I said; zero percolation issues with 87E10.
Jus trying to help.