I've been having an issue with the ignition switch, and I can't figure out what the problem is. The first problem was the ignition cylinder would come out when I pulled the key out. I couldn't get it to lock into the ignition switch. So I bought a new ignition cylinder. Same problem, it just slides in and out, but won't lock in. So I tried to replace the ignition switch itself. The one I got from MU for a 66 didn't accept either of the ignition cylinders that I have (also for the 66) 
The reason I'm trying to replace them is that when I tried to start the car, I cranked it over, and even after I turned the key to off (making sure to hold the ignition cylinder in) the car kept turning over, until I ran over and disconnected the battery cable. It did this a couple times. Any idea what could cause this?
The car is all stock, in terms of the ignition. Any help would be appreciated. Thanks.

The reason I'm trying to replace them is that when I tried to start the car, I cranked it over, and even after I turned the key to off (making sure to hold the ignition cylinder in) the car kept turning over, until I ran over and disconnected the battery cable. It did this a couple times. Any idea what could cause this?
The car is all stock, in terms of the ignition. Any help would be appreciated. Thanks.
You can "Geek" all you want, the fact still remains that when you try to crank an engine with a low battery, the solenoid will stick while cranking, even after the battery's voltage is too low to then make the solenoid engage the starter (if you tried it while the battery's voltage was too low at that point) A 9V battery doesn't have sufficient amperage to engage an automotive solenoid or starter. I've seen this happen many, many times in my 30+ years of wrenching.
Take any battery, drain enough juice from it, then try to start an engine (remove the coil wire to avoid it starting for the test) Keep crankin till the starter drags down and 9 times out of ten, the solenoid will stick open and the only way you'll have to disengage the starter is to remove the battery cable.