These cars used a fairly simply check-valve on the fuel pump to prevent fuel draining back into the tank when the car was parked. As the car ages, this valve wears to the point where it won’t maintain fuel in the lines going to the engine. If that happens, it will take some time and cranking to get fuel from the tank to the engine. This is the delay you’re experiencing.
The worn valve is only allowing a small amount of fuel to drain back which is why you need to wait six hours or so for the problem to show up. A new fuel pump will fix it.
