Ben,
We have files from our LCII that have been caried forward to a G3, G4, G5. and then an Intel PowerBook. The one constant has been to open files with Text Edit on the G5, view the document to check for content. Then save the file as a text file with the letters ** for back up. Then open the file with Word 2004. Word will not open the file cleanly unless it has been automatically dressed up by TextEdit. Different software will view the same file in different ways. The key is to make a backup of the file, then play with "Save As" using different types of software.
Each generation of software will only look back so far. Testing different software and routines will help you arrive at one that works. Back in 1990, we used to open files in Word to view and make line return additions or deletions, then open the file in Works to preview and save, then open them in Filemaker to use the data. Each software title had a feature that was used to automatically get the data to the next stage without a lot of manipulation or data entry.
Where in the world are you? Name a large city near you and maybe a user group can be found. User groups can help you find old-timers who still have older machines that will bridge from one generation to the next. For example, we have an Apple IIc with a uni-drive. Pro-Dos files can be moved from a 5.25 floppy to a 3.5. That can be put into an SE all the way up to a G3 Platinum. From a G3 you can go anywhere.
A combination of hardware options and software generations will allow you to still read files from 1980.
Ji~m