1964 Pontiac GTO won’t turn off.
So we’re restoring this GTO, when I take the key
out the car will continue to run. Can’t seem to figure
out the issue, any ideas. Already replaced the
ignition switch. (this model has a 4 barrel engine)
Problem might not be in the ignition switch, but in the cylinder. Might be a defect in fuel delivery or spark, like a sticking relay: the engine continues to run as long as air is entering the intake. If it doesn't sound like it's running normal, but Dieseling, there's something in the combustion chamber that's overheating, like plugs that are the wrong heat range, or the timing is sufficiently off to cause heat retention inside the cylinder.
Is the alternator stock or did you put a one wire system in? In my 68 mustang I have a one wire alternator and sometimes it will keep juice flowing to the coil after the ignition switch is turned off. Good luck.