I need some Mopar help

My nephew has this 2000 Dakota 4x4... 5.9 Magnum, 180K when the oil pump failed and took out all the mains plus 3/4 of the rod bearings.

I go on Craigslist, and find the only reasonably-priced replacement for it, out of a '97 Ram 1500. Upon stripping it down for cleanup/re-gasket/paint, I discover that it's actually a 5.2.
Whatever, let's just get it in there and we'll deal with whatever comes up when it's running... fast-forward a couple weeks, since I work overnights at the hospital datacenter and can only really work on it once a week, and it's all installed and we get ready to start it. It fires right up, it wants to run. Let it idle and plug up the couple odd vacuum leaks that occur, and it smooths out, holding 650-700rpm, no weird noises, steady exhaust tone, no white smoke or any other signs of trouble.

And then, after about 10 minutes, once the temperature starts to really come up, it starts PUKING black smoke. Like, it filled up the 40x60 garage in about 45 seconds even with the exhaust fans blowing. So I shut it down, and start checking sensors. O2 is plugged in, MAP tests out within spec at 0/5/10/15/20"Hg, but the top of the ECT sensor breaks off as I jiggle the connector. No problem, I run to Autozone, grab a new one, test it on the bench and throw it in. Restart, now that it's cooled off for two hours, and it idles fine, no problems for several minutes, until it warms up and again starts belching fuel-rich exhaust.

This time, I unplug the O2 to see if that makes any difference, and it does not. My co-idiot friend thinks the problem is related to the fact that it's got a 5.9L PCM and is just dumping too much fuel... to which I point out that the difference between a 318 and a 360 is negligible, and that a 90cc-per-hole difference isn't going to make it dump so much fuel that it nearly puddles on the ground at the tailpipes.

So now, we're at the point where we've built a circuit with a potentiometer so that we can manually tweak with the MAP voltage and see what happens, but I'm not convinced that it will make any difference. The EFI on this thing is painfully simple and it's pissing me right off that I can't figure out what the problem is...