If it were mine, I'd pull the engine and replace them all at once.

As long as your exhaust isn't rusted solid, you should be able to pull the whole motor in about 3 hours, figure an hour to do the plugs, and another 3-4 hours to reinstall everything.

If you've never done any heavy maintenance in the engine bay, then double the time estimate as you'll be fighting sized up bolts and stuff. Regardless, you'll be able to start on a Saturday morning and finish it by Sunday afternoon, and they'll all be fresh leak free plugs.