['65 Super Sport 327 700R4]

I am sort of lost here, I have 4 buddies that have '73-'80 Chevys and half of them have cats and half don't. The ones that don't have them are a '78 Camper Special with a 350 and a 4-speed, and the other one is a '74 C10 short bed with a 350 and TH350 trans. The ones that have cats are a '75 C10 "BIG TEN" short bed with a 350 and TH350 trans, and the other one is a '78 C10 short bed with a 350 and TH350 trans. Why do some of them have catylitic converters and others don't.
I have a neighbor who had an '85 C20 scottsdale that came with a 6.2 diesel, but it was replaced with a 350. He doesn't have to go through emisions because it was a diesel. I am thinking about picking it up and dropping in a stout big block, with a carb and no emisions controls.

Thanks,
Erik

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