Engine will fire than shut off

[1968ls2fisher]

What's up guys I have a ls2/t56 magnum swap I'm finishing up. I have the motor installed and most of the wire connected. I'm running a reworked stock harness. With 2 12v ignition wires 1 12v constant wire. I'm getting the 12v ignition from the fuse panel under the dash. And the 12v constant im gettingstraight from battery. The engine fires for few seconds and than dies. Any suggestions on what it might be and things I can check on. Thanks.