[HO455]

Try putting the transmission in drive and see if it happens. If not then take a good look at the connector on the starter switch. They are a known failure point.

If so then you have some wires shorted together after the neutral safety switch and before the solenoid.

And ignition switch failures are pretty common especially with some of the parts sold today. For more ignition switch information see this thread.

https://67-72chevytrucks.com/vboard/...d.php?t=818861

And as I said on the truck side "Let us know what you find."

With more information I came to a different conclusion here. It may be that both the starter and the switch are/were bad at one point