1979?
Imagine what you do to keep your 1stgen running. Now add another 10 years to its age. Now add ~75% to the cost of all repairs.
Now depend on it to make money.

It's the same story though, as long as you can work on your own truck I can see it being somewhat feasible. But then you'll need another set of tools, 1" impact, 3/4" socket set, blah blah blah...
I do respect seeing a nice, old truck out doing work. There's a guy that comes into work with an old Louisville tri-axle that is spotless, shiny wheels, clean paint, you can just tell the dude goes home and washes it every night. I love that truck, it brightens my day just to see it roll in.
You think it sucks keeping a 1stgen on the road? Try keeping a spotless 1stgen on the road.
What's the GVWR of a 1stgen? What's the GVWR of that K-dub?
Take that multiple and apply it to the upkeep. I'd guess that number would be pretty low.
