I'm not an expert in jetting a carburetor, as a matter of fact, I know diddly about jetting a carb, but I'll bet I can help you with other things pertaining to your emissions. I doubt that it being overjetted is the sole cause of it being deemed a "gross polluter." Did it do worse with the HC emissions or the CO emissions? You mentioned ignition timing, which is a big deal, but is your EGR valve working? You can test it by pushing the plunger up, capping your finger over the nipple, letting go, and seeing if the plunger holds. If it's a newer valve, it may be harder to do because their plungers tend to be encased in a clamshell rather than being open like the old valves. Is everything for your A.I.R. system in place? I'm assuming so because of the visual component of the inspection. I would inspect your canister and make sure everything is hooked up, and the vacuum lines aren't falling apart. There's likely a filter under the charcoal canister, too, which you should change if it's looking not so good. Just unbolt it and flip it over to see. It may be a good idea to throw a new fuel cap on there if it does have an evaporative emissions system. It may also be a good idea to freshen up the plugs if they're looking tired, and the cap and rotor can get tired, too, if they're old or well used. I wouldn't worry about the plug wires at the moment unless they're chewed up, falling apart, or corroded on the inside. Finally, pull the PCV valve out, shake it, and if it rattles, you can keep it, but if not, get a new one. I'm not totally familiar with CA emissions components for a '77 truck with a BBC, but that's a run through of some emissions failure points that come with age, which I assume your Burb has many or all of said components.
