Starting and idling problems

I'm going to order an electric gauge and run a sender so I can permanently monitor it if needed.

Good news though, after doing numerous tests checking volt drop at the fuel pump etc thinking perhaps it was getting full voltage and therefore not pumping out at its optimum pressure. I found nothing out of the ordinary I thought, look, I'll swap the 5.7s back in again and see what happens , fully expecting nothing to have changed. Sure enough bam fires in to life beautifully! I don't know if there was an air lock or some issue when I had taken the injectors out and left it to sit for a week? But after running the 6.1s and then putting the 5.7s back in She now runs sweet as a nut. I am still going to get he permanently mounted fuel pressure gauge for the dash though so I can monitor and help diagnose in the future if anything odd occurs again.

Thanks to all those that chimed in with ideas and things to check