1971 LS5. Was working fine. No issue, no warning sign until the engine just stalled in the last turn, 100 yards away from home.
Then I remember that, at a traffic light 10 minutes before, the idle dropped to 600 rpm until I added a little gas and it went back to 1100 rpm (normal). So, I suspected my gas filter might be clogged. I then added some gas directly into the carb, thinking it would be enough to get to my garage where I have the tools. Tried to start (normally it fires up right away when I add gas into the carb).  Battery is fine but it won't fire up. Nothing. Not even an hesitation.

So, if it is not the gas supply, it must be an ignition issue. If my battery is fine, that means that the alternator works, right. I checked the connection of the distributor and the solenoid. Everything looks fine. 
I have installed the Pertronic Ignitor III almost 5 years ago and it worked great. Is this possible it just dies on me without warning? Where do you suggest I start?

Fuel filter maybe.

I've had a drag racer tell me Pertronix is junk, so maybe look there. 

Thanks. I eliminated the fuel filter as a cause after failing to fire up the engine with gas in the carb directly.

As to the Pertronix Ignitor III being crap, I am not sure. It has worked beautifully for almost 5 years. This does not mind it can't fail. I removed it and re-installed the points I have kept from 5 years ago. Not much luck.

I'd still replace the fuel filter.

Coil?

Fuel pickup in the tank clogged?

Wires to the disty?

Is the rotor turning with the engine?

Just spitballin' here.

In the process of checking your connections, did you happen to actually check for VOLTAGE at the coil while cranking? It's possible you have a fuselink that crapped out, and now you have no voltage for the ignition. Try running a temporary hot wire(12v) to the positive(+) side of the coil and see if it will fire.

Gary had a good point too...is the distributer rotor even turning while cranking? Might have sheared the drive pin on the gear, or the gear could be wore slap out where it runs on the cam.

Yes, I tested the voltage at the coil while cranking (with the help of my wife) and I got nothing.
I did not know there was a fuselink on that wire. I will check. Thank you.

And yes, the distributor rotor is turning while cranking. That, I had checked.

So, now we know there is no voltage at the coil. It's a starting place! You should have a yellow wire that comes from the "S" terminal on the starter solenoid(outside, small terminal) that goes to the + side of the coil. This is your full 12v source for the ignition while cranking/starting. Another wire on the + side of the coil comes from the ignition switch that powers the coil after you release key from the "Start" position. This other wire, IF original, is a resistor wire, and it cuts the ignition voltage to the coil to around 8-9v. This keeps from burning the coil and points out quickly, as they would with full battery voltage all the time. Since you have NO voltage while cranking, I would have to suspect an issue with the yellow wire coming up from the starter solenoid, or the solenoid itself("S" terminal might not be making contact internally while cranking). Since you said it started running a little rough before finally conking out, I might also suspect the coil. 

With the key in the "Run" position, you should see anywhere from 6-9v on the + terminal on the coil. If not, you have an issue with the resistor wire, or whatever wire is now coming from the ignition switch to the coil. There should be a fuselink in that circuit, on your '71 it will probably be somewhere close to the horn relay on the driver side fenderwell. Should be a red wire with black stripe & white fuselink...which is prolly more yellow/tan by now(age).

Entirely possible a bad connection on the fusebox pass-thru(fusebox connector). I think I would first jumper 12v to the + side of the coil and see what happens. See if it will fire off at least. If it starts and runs, then you need to repair the circuit from the starter solenoid to the coil. If it starts, but then dies when you release the key, you also have an issue with the wiring from the ignition switch. 

hth

btw...I have the wiring diagrams on my webpage if you would like them. You can view or download from there if ya want.