Is 350? hot enough i.e. within spec?

You already have the usual suspect in mind- the thermostat. "1/8" above the cold mark" is too cool. If a car runs too cool it tends to throw the sensors off and run rich. My 1989 3.0L had that problem, running too cool (wrong aftermarket thermostat) and crud accumulating in the combustion chamber.

The no. 1 suspect is your state government. Does AZ have smog referee's like California? If so, drive over to the referee and have HIM/HER tell you what is wrong with your vehicle so YOU can COMPLY. Damn, AZ should at the very least do that. Or cross the border into Cali, have a smog test flunk and then go to a Cali referee tell you what's up under California's exacting standard.

No. 2 suspect is a cracked vacuum hose. No. 3 suspect is the control of vapors: fuel filters, evap canisters, etc.

You are going through the problem sequentially, which is good.