I am guessing it is the ignition key switch but what can be wrong with it?
I am starting the car fine no problems but when I turn the key to the off position car keeps running smooth as silk. It has never had a problem until now.
Any suggestions on what to do to fix it.

I'd pull the wire on the back of the alternator off while it's running with the key in the off position (VERY CAREFULLY). If it quits, the alternator is back feeding the ignition. If it keeps running, it's something else. It would be rare for it to b e the the switch. Are you sure you didn't change anything recently, like the alternator????

Nothing has been changed. I also wonder if I could have a short somewhere?

I will pull the wire on the altenator.

but when I turn the key to the off position car keeps running smooth as silk

Slowly continue turning the key towards the accessory position, does the car shut off?

Most probably the alternator back feeding through the ignition switch. Need to install a diode to fix it if that is the issue.

Mike

OK, let's go back to basics. Put a meter on the power lead to the ignition. If the ignition switch off you should have zero Volts. If it's on you should have 12 Volts. Do this step by step. If the Voltage is off with the ignition switch off, it's not the switch. Next start the car with the alternator disconnected. If you turn off the ignition the car should stop (it's running only on the battery). Reconnect the alternator. Start the car. If it runs on after you turn off the ignition, there is a feed back loop that is allowing Voltage to keep the car running and it's coming through the alternator/regulator circuits or you have a shorted diode (not likely because, don't most diodes blow rather than become bi-directional???? Maybe the field and sense wires are reversed?

There are other who know more than I, but methodically testing, step by step, is the way to troubleshoot. If you eliminate everything else, what is left is your problem.

Chuck

i had same prob turned out to be a blown fuse after changing some guages

Installed a universal painless kit on my '64. Car did the same thing....had to pull the wire to the alternator. Connected a diode and walah...problem solved.

turn your key back while twisting the steering column back and forth. once you get it to turn off leave the steering column in that position.