Corny but the plan worked for us in Colorado. I believe as others noted immediately above you will need a cat, that is most of their visual inspection, as well as unleaded fuel reducer in the gas tank filler! FYI for the other suggestions to grease palms etc. In CO it’s (now) a state run system “Air Care Colorado”; it is less likely to “grease a palm” (and fairly crazy to try to bribe a state employee or agent).I think the cat idea is a good one and will provide a measure of control. If theyreally wanna see a visual….grab a bunch of plastic-T fitting and Y fittings…connect them all within a foot of each other with 5/16 black hose and terminate the hose ends in a spot that’s hard to visually access. Lay the T’s and Y’s all over the top of the engine in obviously ugly places…
Corny plan but while you're explaining the catalytic converter they can see, you distract their visual scan towards another conversation…I have seen dumber ideas work
But as stated if they see a cat and a bunch of vacuum line they are likely to say “whatever” if it passes tailpipe (again a low bar in CO for 1970’s cars). It worked for us on (now grown) son’s CJ7! I just made sure the vacuum lines and wire terminated someplace, heck maybe some even terminated in the correct places but there was no vacuum. The classic plate last for 5 years and we need to go through that again

