I've got a 89 Formula 350 w/90,000 miles (~25k on the engine and 12k on the trans).
I'm going to have a set of hedders and new cats installed, as well as replacing the spark plugs, the O2 sensor, the spark plug wires, cap & rotor, and was wondering if there are any other components that usually need to be replaced at this amount of mileage, that would be easiest to do when installing and replacing these components. Thanks.

 

If you're installing headers, anything is pretty easy in comparison. You could replace your ignition coil, relays, various sensors. There are numerous things. As long as the car runs fine, though, you really only need to watch and make sure it stays that way.

 

By the way, here is a nice chart Vader posted: