Smaller model GMC trucks in 1958 had an ID plate on the left door hinge pillar that had the serial number and engine ID information. I don't know if that carried through to larger trucks like yours. Larger trucks had a similar plate on the inside of the glove box door that had the same info as the tag of the pillar plus additional info about the transmission and rear end.
Below is extracted from the 1955-60 GMC Master Parts Book printed 12/60 and covering 100 series (small pickups) through the largest GMC trucks made during those years . The "S" series letter defines 1958 and 59 models. I have the book in both paper and .PDF form. I don't remember where I got the .PDF file, but if you are interested I could try to find out.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
