Looking at a picture of Burts truck, it looks like he has a "full convertible" removable top, which would have been a K5 Blazer/Jimmy before 1976.
In 76 they went to the half-cab design, so if it is a 79 he probably would have had to modify the truck a bit to achieve the "full convertible" look he had going.
You have a good point. But, there is a possibility it was modified because the entire truck looks custom! The paint job, the chain rack, and the gun rack. So the roof might have benn removed.
I have El Sonoma del Torra de Fiero Syndrome. Be happy you don't. Trust me.