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.
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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)