Ignition Issue, motor wont stay running

71 Duster, Got a 360 magnum swap, hei dsitributior factory collumn, carbureted.

Motor fires right up as soon as you turn the key to engage the starter, but if you let go of the key it instantly dies.

If you hold the key in the start position the motor will keep running as long as you hold it....

Any idea what wires might be switched/crossed?