Maybe you already know this, but the part that threads into the mixing elbow is a left hand thread. The part that threads into the manifold piece is standard right hand thread.
In my case after the short coupling corroded through, I had to remove the piece from the manifold side with a small chisel. I tapped with a hammer around and around until it collapsed into itself and could be pulled out with a pair of pliers. With all the exposure to heat it is well annealed and therefore rather soft (ductile). Since we were in the middle of leading a Club cruise on last year's Forth of July weekend when this happened, the proper replacement parts were not available, so a quick fix solution was taken, which worked quite well. I still have that part for an emergency, but did replace the elbow and coupling when we returned.
I also use anti-sieze on the bolts before threading them back into the engine block. I have heard stories of them being stuck so badly that the heads sheared off when trying to remove them. Imagine trying to drill out the remaining piece of bolt in that situation.
And here it is the Fourth of July once again and we are leading the cruise from San Francisco to Monterey starting Saturday. This time, we should not have any issues (at least with the manifold).