I changed alternator on my '71 442, and now the key doesn't always shut off the engine. The gen light is working as well as the rest of my electrical system..The new alternator checked out good, btw.

 

Well that's a new one! I changed my alt last year to a new 55amp. Direct bolt in and no problems. I would guess that you have some issues with your ignition switch. I seriously doubt that changing your alternator would have any effect whatsoever with your described problem. Did you check all the connectors at the bottom of the steering column to make sure they are tightly connected?

 

Are you running a MSD box? I have seen some cars you had to run a diode in line on to get them to keep them from running with the key off.

 

Here's how I have things wired; wire from horn relay to a Painless Cirkit Boss;this wire powers hot all the time circuits; the keyed circuits are powered by the IGN spade on the fuse panel; one of the keyed circuits goes to the coil. So, why isn't key off taking power away from the IGN spade? BTW, I've had this wiring setup for years, only change is alternator.

 

After further investigation I noticed that the key wouldn't quite go all the way to the off position. I dropped the column, and after removing the ignition switch the key still stopped short of off or accessory positions. I'll report back after my trip to the steering column repair guy.

 

Hmm, just came across this, not sure if you were looking for me or another Rob...
Either the linkage in the column is messed up or the ignition switch itself is bad, sticking, or somehow shifted.
As the the painless thing, painless is often an oxymoron...