What’s my chances of being able to replace the crossover piece and gaskets without damageing the very rusty manifold.
Your chances are as good as anyone’s. The manifold is pretty sturdy, despite the rust. I’d spray lots of penetrating oil on any bolts connected to the manifold and give the oil some time to work. Multiple applications are in order here, and patience.
Use quality 6 point sockets on the bolts holding the pipe to the manifold. And as previously suggested, use plenty of penetrating oil. These bolts are the most likely point to give you trouble.
Note that WD40 is NOT penetrating oil.