Talk about bringing back the dead.
Here's some info from our friend KIILew, thanks again Ken.
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For 1973 thru 1974, no converters on any models, anywhere.
For 1975 thru 1977, catalytic converters were used on all C/K 10/15 trucks nationwide with GVW's under and up to 6,000 lbs, except 1976 and 1977 C10/15's with the 454 V8. Models with GVW's over 6,000 lbs (i.e. all 1975-77 C10/15's with the F44/Big 10/Heavy Half, all 1976 and 1977 K10/15's and all 1975, 1976, and 1977 C/K 20/25/30/35's) did not use catalytic converters.
For 1978, all C10/15's with GVW's under 6,000 lbs have converters regardless of engine. All other '78 models (i.e. Big 10's/Heavy Halfs, K10's, C/K 20/25/30/35's)had GVW's over 6,000 lbs, and therefore did not use converters. EXCEPTION: For 1978, all California models with GVW's under 8,500 lbs had cats. Therefore, this California exception included ALL and ONLY 10/15/20/25 series vehicles, including Big 10's/Heavy Halfs.
For 1979, Federal laws, like California statutes in '78, raised the GVW threshold. So, all 1979 models, except for C/K 30/35 vehicles, had cats, nationwide.
For 1980 through 1986, the same models as those for '79 used cats. However, a new C6P heavy duty option was offered for C/K 20/25 models which provided an 8,600 lb suspension package. These new, heavier 3/4 tons cleared the 8,500 lb limit, and so did not use converters during this time period, nor did C/K 30/35's.
For 1987 and beyond, all Chevy/GMC fuel injected engines used cats regardless of location or weight class. Only a few 30/35 series chassis cab models offered carbureted 292, 350 and 454 engines, and those were the only models that did not use catalytic converters for 1987 on.
Note: The above applies only to U.S. vehicles. Canadian Chevy/ GMC trucks followed a much simpler story, I think. From what I have seen, no Canadian carbureted Chevy/ GMC C/K/R/V used cats, while all '87 and later fuel injected models did.