That can be ruled out. The tank was dropped, emptied, flushed and prepared when the electric fuel pump went in.
Back when I put Holley Projection on Warhammer I did what you are doing, hung an electric pump on the end of the pickup.
Now the factory did that to feed a carb and deal with vapor lock issues, but with fuel injection it doesn't work well. As someone mentioned, at half a tank it'd suck air as the fuel sloshed away and it'd kill the engine when I made a spirited turn. The lower the fuel level got the more it appeared. Now I know you said the tank is full and probably isn't the issue, but I went thru a couple of burned out pumps dealing with this. If you get it on the road you might want to put in a hydramat on the end of the pump, at least. I ended up going with a fuel sump fed by the mechanical pump, that in turn fed the electric pump. Something to keep in mind is all.
As for you original question, use the ballast feed (12v side) to trigger your pump relay. Since if that is flaky your ignition would turn off too and it doesn't sound like your having an ignition issue.
