Thank you for your quick response! I will check out those links. I modified my original post to clarify that the engine is a rebuild from an old truck of my dad's. Thanks again!

I have a '76 F-250 High Boy. ( I rebuilt the engine from a '68 390) I recently replaced the solenoid after checking the starter and finding nothing wrong with it. It fired right up! No problem! Drove it around and it wouldn't shut off. Turned the key to accessories, pulled the key out= nothing...