chevydekotah

I have a 95 chevy 1/2 that is my mud truck. Had a 350 tbi in it. Pulled it out and swapped in a 97 vortec 7.4 with a 4l80e tranny.Put a new vortec big block fuel pump,got 65 psi to the rails. Replaced the oil pressure sensor that I broke when it was installed. New crank sensor also.swapped the wiring harness in. The truck starts right up no problem. Sounds healthy. But as soon as I stop cranking the key, it shuts off. Cuts out. I had it running for 5 minutes when I pulled the relay for the starter,so I wouldn't burn the starter out. But I had to hold the key forward the whole time. Any ideas?