Hi Bill. In my mind I am right there with you, on the floor and exhausting the four letter word vocabulary. However, I see myself looking at a crossmember that is in a channel cofiguration. On some of them the channel is open to the top, others are open to the bottom. Recently I have done some iron work on other projects and after all these years I have discovered the old side grinder with a thin metal cutting blade. To cut steel angles and flat bars I used the “Sawzall” with that little blade going back and forth and using some more of those words. Well, the metal cutting blade in the side grinder has taken care of all that. Try it. You might even think to cut the cross member not at the point of attachment but more to the center of the car. After getting the transmission and engine out you might have enough room to clean the sides up and install a proper cross member mechanically. All of this is of course the thought of trying to help and feeling the misery. Whatever you do, please make sure that all fuel is removed and there is no chance to even ignite some fumes.
Hope all this helps, but don’t forget to have fun!