Absolutely nothing more than drive it the way it is. Fix the little stuff like missing marker lights and such but nothing more. Unless you plan on keeping it forever don't put anymore dimes into it than you need to because you will never get them back. Turning radius of the Titanic coupled with the ride of Barney Rubbles log car equals not much joy to drive.

That being said I wouldn't part with mine for two good reasons. One, its paid for and cheap to own. Two, I can't give it away. I hear stories about how first gen diesels are desirable and hard to find. Well mine is as rust free as the day it was made and very well maintained but brings no interest in these parts.

I put it on Craiglist a while ago and only got two trade offers for it. One was for a Ford 9N tractor that needed some work and the other was a 70's F250 4x4 that was just a POS. Drive it like it is.