First time I have read codes on a GM, I do believe - was curious, it flashes a 12 at me, then a 13 - and seems to keep repeating. Is that normal?

I know on Mopar they will flash whatever codes and then stop.

Also - from what I found a code 13 means:

Oxygen sensor voltage stays between 0.35 and 0.55 volts for 60 seconds. Engine was at normal operating temperature and had been running for at least 2 minutes after start with a throttle angle above 5%.
That just mean the O2 sensor isn't responding or?

From what I can tell, I'm missing the O2 sensor entirely - it's on the driver's side exhaust right under the manifold on the pipe, right?

It's still running rough, but the plug wires are in pathetic shape, I suspect the plugs aren't any better.

Of all the cars I've worked on, I'll have to admit - it's a solid car, in spite of the very obvious lack of routine maintenence on it, lol.