I'd suggest if you're gonna install a used PI to closely inspect where the aluminum crossover and the underside manifold plastic meet: that's where the plastic corrodes and the small gasket groove deteriorates and breaks off. I've gotten a few miles more out of used ones by taking the crossover off, cleaning up the little rubber gaskets that go in the groove and then putting everything back together with a liberal amount of Super Blue RTV.
And like Davidzq said up above, you have to do the RTV glob on the front bank 1 and rear bank 2 water passages. Be patient and let it cure at least several hours (I've done this job three times now and let it sit overnight each time) before you add coolant.
2005 Lincoln Town Car Signature Limited
5.4 liter throttle body; PI oil cooler, intake tube & air box; Mark VIII aluminum driveshaft; Trac-Lok limited slip; Richmond Gear axle shafts; J-Mod with Sonnax valves; Limo rear control arms and a great sounding stereo that won't ever automatically remember my Bluetooth settings
1999 Mercury Grand Marquis (gave that to youngest son, and he's now a converted true believer)