There were several plants churning out 'C' and 'K' series GMC and Chevrolets according to my Chiltons. I know, not the best source and it's possible that not all plants listed were truck plants.
The abbreviations I see are GMAD, and GMT...which I am going to assume is GM automotive division, and GM truck division. That narrows it down to Flint (Chev plant) Pontiac MI, GMT (possibly both?) Chevrolet Detroit, and Oshawa and Scarborough in Canada.
Really we might be able to get this one nailed down if a few GMC guys posted the build plant and so did the Chev guys. For 81 and up the build plant letter in the VIN is the very last letter before the numerical end sequence starts.
My GMC was built in Flint, which is listed as a Chevrolet plant ('F' is the 11th character)
Anyone with a Chev built in Flint?
Rene