General Motors turned out a fair number of Chevrolet and GMC vans in the 1960s to compete with Ford's Econoline and Dodge's A100. They followed the same formula: forward-control design with the engine located under or between the front passengers, rear-wheel-drive usually, and two wheelbase lengths. A cargo or passenger variant was always available. But I don't think the factory ever turned out anything like this.
I believe this truck started life as a 1968-1970 cargo van with blank sides, possibly a G-20, but has since transcended that form into a cabover truck with a sort of sleeper cab. I have no idea what kind of chassis underpins it, or what powertrain, but the eight-lug wheels look pretty heavy-duty. It all looks tough, though I question a vehicle that appears as though it should have a fifth-wheel hookup for hauling big trailers, yet only has a standard trailer hitch in back. Much of the front end is covered in black duct tape, including a row of vents that should exist on the van's cowl beneath the windshield. Perhaps the strangest addition is a massive sliding glass window at the back of the cab that would look more at home on a house than a motor vehicle.
There may be some creative genius at work here, or some crazy drugs. I'm not sure which. The end result is definitely something unique.