that is mysterious, maybe some crud got into the threads, might need a thread chaser if you ever get it out. One nice thing about most later OMC/Volvo manifolds, they are the same casting for each side, and you have threaded holes on each end. So if the rear drain plug is stuck, just remove the feed hose, might want to remove the 90* fitting to fully drain....
What I'm trying to do, is have a longer set of block drain plugs made up that thread into the standard female fitting in the block. This would make it easier to get at them, for winterizing. Might have to find a machine shop that can do this...
The other thing you can do (CAREFULLY), is heat the cast iron around the drain plug with a heat gun, that might break loose what's holding it, or just drain it the way you did it this time in the future.
The way I got a stubborn O2 senor out of the down pipe on our old Jeep 4.0 six, was to start the engine, warm it up about half way, and go at it with my fireplace gloves on. Came right out! Days of soaking in penetrating oil didn't get it, a bit of heat, did.
