if I dug up enough parts
I would think that if you dug up enough parts, you would CERTAINLY be able to add it on. I have cruise on the 92, 93 automatic, and 93 manual. The cruise function died on the 92 and 93 manual years ago. I suspect that the PCM is to blame on the 92 but don't know about the 93 manual. There were post in the old format about making the cruise work. I just never bothered. A local guy mentioned that his mechanic was in the process of restoring cruise on his truck. Don't know how that turned out.
And yes, the trucks are probable too old to get anything from the dealer. You might get a few repair parts. Salvage yards are probably your only source and you might be on a cold trail.
James