Typically, when ever a Walbro 340 pump is installed, most people do away with the Pulsator Dampner that is on top of the factory pump. Inside this pulsator dampner is a check valve which helps keep fuel pressure built up. The factory wiring setup kicks the pump on for the fuel line prime, then gives power to the pump after the engine has started, or after oil pressure has reached a certain point.
If you remove this pulsator dampner and install the straight piece of FI hose that connects the pump to the hanger, you effectively removed that check valve. The result is, you will see this condition. If the car won't start on the first couple of spins, the fuel pressure will drain back down to zero. My personal car does this exact same thing. Most people have never even seen this pulsator dampner, but it is a small black rectangular piece with a goldish metal top that simply pushes onto the factory feed line on the fuel sending unit, and the fuel pump pushes into the other side of it. Most people remove it for a little bit of extra volume, as well as hack off the feed line tube step down on the sending unit. This does make it hard to start the car when the fuel system looses prime, or if starting a car for the first time after a rebuild and or trying to set your cam sensor correctly after a rebuild. You have to cycle the key a few times to get the pump to reprime.