Below is a list of all about emissions and what years everything happened.  I hope this explains it some and takes out the confusion of it all.  This list was compiled by Ken Lewis.  Ken is an online friend and is a wealth of knowledge on these trucks.  Everyone probably knows him better as KIILew, on the message boards.

Thanks Ken.

Basic Federal (i.e. non California) emission laws are as follows for gasoline fueled vehicles:

For 1973 to 1978 all passenger cars and light trucks under 6,000 lbs GVWR must meet stringent light duty emission standards.  All trucks heavier than 6,000 lbs GVWR must comply with heavy duty emission standards, which are much more lenient.  For 1979 through the current day, the dividing line between light and heavy duty emissions was raised from 6,000 lbs GVWR to 8,500 lbs GVWR, forcing more trucks to comply with the stricter emissions standards than previously.

California emission history is identical with one exception: that state raised the heavy duty emission threshold for California-sold vehicles from 6,000 lbs GVWR to 8,500 lbs GVWR a year earlier, for 1978.  (I.E. the Federal Government followed California's lead a year later, for 1979.)
 

A few other general facts:

Catalytic converters were introduced into the auto industry for the 1975 model year.  From 1975 on, catalytic converters were used on ALMOST all light duty emission standard vehicles (all states) but were not used on any heavy duty emission standard vehicles.  When auto manufacturers adopted fuel injection in their light and heavy duty emission class vehicles, they adopted the use of catalytic converters across the line.  (GM adopted fuel injection and cats essentially across the line for 1987.)
 

What all of the above means for 1973 to 1987 Chevy C/K series trucks follows:

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.