I somehow got to see the picture of the truck. A link showed up at the bottom of my screen. Looks like it is a '60 or '61. Engine is probably a 351 V-6. But you can verify that by the numbers near the water pump. Not sure if any production numbers are out there. You can check Jolly's GMC site for some detailed info on these trucks:

http://6066gmcguy.com/index.php

Edit: By using Jolly's Site and going by the HP and RPM rating from the picture above, you have a 305 V-6 there.

Last edited by C10 - C90 Bill; 03/24/2020 4:07 AM.
'60-'72 Chev/GMC Fan
GMC 9500 Fan
Detroit Diesel Fan