I wouldn't say zero value. I got my purchase price back when I sold my engine/transmission/transfercase (non-running/non-complete). However, if the rust is bad it's a money pit (like mine) that you will never get any kind of return on your investment if you sold it. Right now I'd view it as a functional drivetrain with a VIN.
We're talking about a rig that's 40+ years old. Parts wear out (think drivetrain and other moving parts). Others rust away or dry rot. Some fuse together to make completely different configurations. Unless this vehicle has had a major restoration sometime in its life (and judging by the pictures, it hasn't), what you don't see is going to be what costs the most. This rig obviously wasn't always barn-kept, and likely spent a good bit of time out in the elements. As much as I hate to say it, it would likely be better to part that one out to keep others on the road.
Either that, or decide if you want to nurse it along in its current condition or commit to a major overhaul. Once you start polishing one part, you're going to have to do them all.