I gotta side with Stomper on his quotes.
Here's my take on it:
If you squint hard enough, you can see the rear brake drum protruding inboard a bit. This is a sign of the larger 13x3.5 inch drums which is 1 ton material.
The hub stickout does appear to be a D60. I just can't tell by the hubcap as mentioned before, the 3/4 ton is flatter and 1 ton has a stickout ring, like a volcano.
The frame definitely is '81 to '87 year. The angle for the bumper brackets is the dead give away. The radiator support mount is another. On the '81-'87 frames, the mount is like a box shape. The '80 and older look like a "C" from the front view.
Not all 1 tons had the roof marker/clearance lights. They were standard but they could be ordered without as an option. I've seen a few C/K30/3500's without the lights but most had them.
Another hint: the west coast mirrors. I've never seen them on a 3/4 ton in the '81-'87 years of trucks. Only on the 1 tons.
And I've seen plenty of the WC mirrors on all the trucks from '80 and older. Even on K5's, 10's, and 20's.
Last, the stance. If this is a bone stock truck, it appears to have the 1 ton stance with the spring/frame difference than the K20.
I'm gonna bet this truck is a true K30/3500.
Only surefire way to confirm is to call the guy and get the VIN from him to decode.