Hello Troy. Welcome to FTE!!!
An 84 model may very well still have some natural rubber components, but I don't know for sure. Diesel rated low pressure line (sae30r7) isn't all that expensive so it wouldn't be too big of an issue to replace the supply and return lines. Chances are a $400 6.9 probably needs the injection pump and injectors rebuilt anyways, so that really shouldn't be an issue either.
Sounds to me like all the pieces are falling into place for you.
