EVERY state has vehicle-specific standards. They are much more stringent for modern vehicles than for a 1975 big block POS. 1972s are exempt from inspections, but are still supposed to have factory installed emissions equipment (which in 1972 DID include PCV and EEC, so both the crankcase and fuel tank should be sealed).
Oxygen sensors last nearly FOREVER unless contaminated or broken. I'm still running originals (upstream and downstream) on my Prizm, with 283K miles. Still sniffs at 10% of spec on HC and undetectable CO. The Exploder's isn't original because of an idiot exhaust tech who shouldn't be allowed to touch a stick welder.
And it is S T U P I D to attempt a smog check with an intermittent CEL. OF COURSE it will fail (the CEL alone will do that, but it's THERE to let you know you have a problem burning fuel correctly). You're lucky you aren't a gross polluter now.
It's probably a misfire, but you would be compounding the mistake by not going to a pro NOW.