An L78 car would not have A/C.I think you're looking for the solenoid that bumps up the idle in AC cars when the AC is on. If your car does not have working AC, you don't need it. If it does have working AC, you may have to set the idle a little higher so the car won't stall at idle when the AC kicks on.
Update: Daves7dnova is correct -- the idle stop solenoid is to completely kill the car and prevent run-on. AC cars have a solenoid, too, but it's to bump the idle, not kill it.