I have been working an intermittent problem for awhile on my 82. At times, car will not start. Starter solenoid does not kick in.  Dash lights OK. Starter and battery checks good (done out of car). When car cranks, starts and runs ok. Checked voltage at theft deterrent relay. Good voltage when cranks, but noticed about 9 volts when doesn't crank. Neutral safety and deterrent are ok.  Jumped from battery to solenoid side of theft deterrent relay.  Starter cranked.  So, started chasing things where there could be a voltage drop, focusing on grounds.  Seems like every time I touch something, car cranks. Jumped from frame to starter. Cranked a few times, then same problem. Cleaned and redid frame to engine ground, and battery to frame ground - ok for while. Although a long shot, changed the fusible link on B+ terminal of starter.  Then because I had two fusible links, changed link that goes to alternator.  I am going to continue redoing ground points.  Any other suggestions?  Bottom line seems to be there's voltage, but at times there's a drop somewhere that prevents starter from cranking. Car is looking real good though as when I'm frustrated, I clean it. So, the underside is looking just as good as the body.

Looks to me like you have either a problem with the ignition switch, or the theft deterrent module...

You said you checked the battery out of the car.  Might want to check the vltage in the car when it won't start.  Just in case there is an intermittent short between two cells.  Just in case, to rule it out...

Every time I've had that problem with a Chebby, it is the solenoid itself on the starter that is bad. Works sometimes and don't even get a click others. Replacing solenoid or starter solved the problem all the time.