So here’s a bit of an update. I have the manifold sitting vertical, with the adapter on top. Every few hours I’ve been heating the manifold with my map gas torch at bit, then squirting in some penetrating oil where the copper gasket use to be. I’m hoping the repeating heating will both expand the aluminum around the fitting, and thin the oil, with the combination of those allowing gravity to pull the oil down into the threads.
I also made a run to HD and bought a 12 point std socket and a 6 point deep impact socket. At some point I’ll grind down the 12 point and see if I can get some movement. I may swing by a friends who has a long breaker bar too…
Wish me luck!