Finally, some good news.
Long story short, there was still too much corrosion inside the aluminum headers, where the oil/coolant O-ring are. A small die grinder cleaned up the aluminum oxide and then I used fine sandpaper and scotch bright to finalize the finish. I was originally too afraid to use such aggressive methods but since there was nothing left to lose...I let my Dad try (yes, his idea, and he saved the day again).
Once assembled, I submerged the assembly under water and pressurized with shop air. Previous tests had a bubble come out every 5 seconds. But by morning of the last try, it was still air tight. No sealant on the O-rings this time, but I did use it on the gaskets (which I made from scratch since I had no more spares). The new cooler bundle is in, but I guess the old one is still good, other than some corrosion at that O-ring location.
I pulled the cooler three times over the last several weeks before it finally stopped contaminating the coolant. Every time, I had to separate the oil out of the coolant all over again. Over the last week I could see the oil gradually clear up from the rad until it finally burped the last of it into the expansion tank. There's a lot more to the story but honestly I'm just glad it's over.
Now that I know the truck will live, it's time for an oil change.
Whew...
