While doing the emissions inspection on my '76 (2.0 GC) I noticed the mixture was very rich, fluctuating between 3.5-4% CO at idle, I believe it should be about 2.5%. Which way do I turn the ECU knob to lean it out? I thought it was counterclockwise - but it's already as far as it can go... (unless I am wrong and the CO is actually OK, which I doubt)

Yep its rich. CCW should lean it out, but you've already pegged that full lean. Maybe check the other sensors...TS1, TS2, TPS calibration, consider futzing with the TS2 resistance. Should be 2.0% +/- 1..0% Is your catalytic converter still good or do you run one? See the emissions labels for both CA cars and 49-state cars below:

Thanks for reminding me to look at the dang sticker, I always forget it's there for such information and keep looking for various sources on the net which invariably quote a mishmash of half-correct information

The exhaust system has not been changed and it has a catalytic converter. There's no indication that it's broken in any way, but since I did measure in the correct place using the access hole in the first muffler the catalytic converter shouldn't influence the measurements. I did not disconnect and plug the crancase ventilation, air pump and evaporative system hoses, though. Maybe that's the problem.

Is there any obvious indicator for TPS calibration problems? The sensors are a simple resistance measurement but I'm less sure about detecting TPS problems, short of just going through the full adjustment procedure.

If the TPS is misadjusted, it may stumble right at the transition from idle to acceleration. The only reason I suggested that, was at idle two contacts in the TPS tell the ECU to control idle mixture through an idle circuit and off idle other circuits in the ECU control mixture.