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

A long hot-cranking time, speaks to a lack of combustible fuel reaching the cylinders.
One would think that you should be able to overcome this with either; pumpshot, choke, or wet fuel level.
However; if the exhaust valves are not sealing properly, "suction" at the carb will be poor; and/or if the intake is sucking dry air between the carb and the chamber, again, suction at the carb will be poor.
Or if the brake-booster diaphragm is perforated....... or the PCV is internally busted, the hose is ill-fitting, etc.

My test is to put my hand over the carb during cranking, and have the engine try to tear it off my arm........ This action should flood the intake with fuel. As soon as you get your hand off the airhorn, the engine should begin firing. It may subsequently stall or just quit firing, and not start, but it sure won't be from lack of fuel, lol.

The other thing is this; if your throttle is too far closed, the transfer-slots will be nearly blocked off. How is the engine supposed to start in this predicament? Those transfers are your primary slow-speed fuel suppliers. The Idle mixture screw(s) are just trimmers. In this predicament (too-far-closed transfers) air, at idle, will purge the idle-wells and dry them up. No surprise that the engine will not restart.
I'm not saying your throttle is too far closed, only that it might be. This is usually a by-product of too much idle-timing. With extra timing, the idle-speed automatically increases. And so as it usually goes, your tuner just cranks the speed-screw slower. And of course, the engine is now struggling for gas, so the fix is to increase the fuel coming from the mixture-screw(s). So it idles . Whoopee. But now, as soon as you tip the throttle in, she goes rich, and stays rich on that circuit....... cuz the mixture-screws are too far open.
My starting point for the mixture screws is to idle the engine up to around cruising rpm, then close the trimmers as far as is consistent with a smooth steady rpm. After that, I slow her back down to idle, and adjust the idle smoothness with the TRANSFER fuel. If the idle-speed is too fast, I take out timing.
Sometimes I fine-tune with the wet fuel-level.
I always run my stock slanties with what most would say is baggy valve lash, namely .013/.023 @ an adjusting temperature of 75/85 air temp.