Below is the listing for 6 bolt Budd type wheels from the 1955-1950 GMC Master Parts Book. Your '58 would be an "S" in the "year series" column. Look for your serial number on the tag on the cab's left door jamb. There may also be a tag with the serial number on the inside of the glove box door. In either case the serial number will start with a 3 digit number that is the model number. In some cases of larger trucks there may be a letter code before the model number. In either case, use that information to look up the proper wheel type and dimensions in the chart.
By the way, "Viking" is a Chevy model, not a GMC.
'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.
