* spinedoc, did you buy this car (R129.067 w/ M119.972) before it had, say, 50K on the odometer? If you did, I don't understand the sludge with 5K synthetic oil changes. You know that despite the fact that grannys can drive very slowly and carefully, they routinely forget about maintenance (the real saving grace is the lower mileage, not the maintenance record).
* You absolutely need to find not only the noise-creating DAMAGE but also the CAUSE for that damage in the old engine before you start up a replacement engine. All you've found is the sign of the damage, the fine metal "slurry" or metal chips (which is it?) in the pan. Unless there's damage on rod bearings, cyls 7 and 8 (you've checked all the rest; how about a couple of main caps?), you don't have an oil-pressure related problem. Besides, I understood you to say there was oil pressure, possibly low-end of pressure range, but still acceptable. If the oil pump stops delivering oil even momentarily at road speed, there'll be bearing and pump damage, guaranteed, and a clear pressure problem hot. There are several soft bushings (timing chain idler) in that engine which could disintegrate and leave metal chips in the pan, not affect oil pressure, and cause some noise.
* If the damage turns out to be fuel-wash related scoring, make sure you cover the cause of that fuel delivery (electronic? or just mechanical pressure regulator). Don't carry the cause from the old engine to the replacement.