What are you after? more power? more MPGs?
for power you can take a look at the exhaust system and see what size it is. 3" is a good size for a non turbo 6.9. 2.5 is too small and 4" is a waste of money unless you run a turbo. Then look at the intake and see if there is still a big bowl hanging down from the air cleaner lid. If you find one, cutting it off will gain you a slight boost.
Assuming you have stock intake and exahust, you can get a "SLIGHT" gain in power but its not going to be a night and day difference.
If you want a night and day difference, then you need to look at a turbo. New that start around $2000, used you can get them anywhere from $200 up to $1000 typically. Lower priced ones usually missing more parts and needing more work. If you already have one, then use it. None of us ever regretted adding a turbo.
Turbocharging won't make a noticable improvement in MPGs in most cases, but the truck will pull better for the same amount of fuel. The catch is being gentile with the pedal enough to get that extra MPG or two. Most of us are not able to do that
Other then that, just keep up to date with the oil/filter changes and never sell it. 19 MPG is great for that era (no overdrive).
