Slant 6 down on power.

1. As said, start with the ignition timing.
2. Then run a compression check on the cylinders.
3. Adjust valve lash.
4. If it is running unevenly, shoot some valve cleaner, or carb cleaner followed by oil, at the valve stems through the springs to loosen up any gum in the valve guides.
5. Go through the detailed process of checking the cam timing. Just a worn timing chain will cause these engines to run sluggishly. If this engine has a lot of miles, replacing the timing chain is a good idea anyway.