I have a GMC 250, it’s a one ton. If the wheels are 20” I’d say that the truck isn’t a 250. At this point the hood could’ve been changed 50 or 60 years ago and hard to tell if original.
The above suggestion– check out oldgmctrucks website. Doesn’t take anything away from the Stovebolt but specializes in exactly these heavier gmcs. It’s a slow forum but lots of good info

1951 GMC 250 Open Express
1968 C10 Suburban
1971 C20 Pickup

My Dad told me "Son, never hit a man in anger- unless you're certain you can get away with it"