Welcome to Stovebolt
Your serial number 3G58T 121677 break down as follows
3G = 3800 (1-ton)
58 = year
T = Tarrytown Assembly Plant
121677 = unit number starting with 10001 (you have the 111,677th unit out of Tarrytown).
If your truck was born a pickup it would be a 3804, the Cab/Chassis (no bed) are 3803 and the stake bodies are 3809. There were no dump bodies offered by Chevrolet, they were all dealer or owner installed.
Mike B
