Can't say that I have a clear answer to your question but I can offer a small amount of info. I sold Greenleaf a crossmember that I don't know if he has installed. I can say that it was quite some difficulty just to get the one he has out of the donor truck. It is a built up assembly that starts with the crossmember and that is then rivited and welded to the frame rails. My guess is that at 450k plus miles, your best bet is to get a really qualified welding shop to repair the cracks and watch it as you run on to some other failure.
As I was torching down to the point of getting the crossmember out, I noticed some other cracks in the frame. It would be a good idea for all owners to get their truck on a lift and clean the crude and look good at the front frane area.
James