I did valve stem seals on mine a couple of years ago in my driveway with the motor in the car. It's doable but it is tedious and frustrating (mostly the putting the valve spring retainer locks back in).
There's not much room to work in a few spots with A/C lines, master cylinders and strut towers getting in the way and it's tight in the heads with the cams and cam towers making access difficult, but, like I said, it can be done if you're patient. I think I wrote up a postmortem thread back when I did it.
Some people say you have to change the valve guides out also or you'll continue to have problems but it's been almost exactly 2 years and I've put on 12K miles since with no more smoking (the car has 137K total). Your, uh, mileage may vary.
I ended up not using anything but new seals. I was going to put new valve cover gaskets on but I was sent the wrong ones and I ended up returning them and putting the old ones back on instead of waiting for the correct ones. New seals, the Trick Flow spring compressor tool, a tube of RTV to help with the gaskets and a compression tester kit from Autozone to use with an air compressor to hold the valves open was about all I needed. I did lose a retainer lock and had to make a trip to the Ford dealership for a few extras.