950 or even 800 is NOT idle rpm for an inline six. Unless you have a big cam, there should be no reason that it would not idle at 500-600 rpm. It should idle well at 400, but oil pressure will drop very low and most factory specs call for around 600.
If it is idling high because of poor carb adjustments, it could have a vacuum signal from a ported vacuum connection at 950 rpm, if the throttle blades are open too far.
It is running when you turn the key off because the throttle blades are open too far and then you are getting pre-ignition from hot carbon deposits, allowing it to diesel.
I would first make sure you do not have a vacuum leak by spraying the intake gaskets and carb base with carb cleaner.
You said you checked the dwell with a dwell meter. You didn't say you had checked the timing with a timing light yet. Check the timing.
Next, you need to adjust the idle down to where it belongs.
