frame
might have been painted chassis [low gloss] black - "driveline" [everything behind bellhouse] generally not painted, nor were the springs or axles .... inner fenders satin black .... engine [usually incl bellhouse] painted a color to identify it, depending on year, not sure on GMC, but light truck Chevs were some variation of gray .... engine 'bolt on' items [genny, air cleaner, fan, etc] gloss or satin black, with some [starter] possibly 'natural' [see '59 car
example here] .... opinions vary, there is no definitive list .... and it doesn't matter that its a 'burb, same as pickups under the skin, might check the Paint and Body forum or look around oldGMCtrucks.com
Bill