I'm curious if you could cut the unibody on two cars and reweld. My thinking is to get a wrecked Honda and cut the front off to put a Honda motor in the Mazda. Its not so much a why should you. More if you could or not. Much like the old ratrods. I was thinking if you could weld it and add some form of subframe support it might hold up.
If you cut the cars in the right places you can (Collision shops do it often enough) But merging the honda with the mazda will require some fancy fab work to mate the sheet metal...